# What would an out-of-water submarine (for water-breathing mer-people) look like?

My mermen & women (fish from waist down and human from waist up, but breathe only in water) are at WWII era warfare and interested in their version of 'underwater,' meaning they want out-of-sea vehicles (sorry, there is no antonym to submarine) to fight over land similar to our attempts to occupy the seas with submarines. I would like it to be the equivalent of a U-Boat / Submarine in an attempt to extend military dominance over land territory. Definitely not air (as in airplanes or jet fighters) .. yet.

The merpeople live between 100m & 1m underwater. They want to have military dominance against other merpeople over what we call land. There are no humans, but there are plants, trees and animals on land, in the sea, and in the air.

The predecessor to this question is a comment on a previous question, in which @bellerophone mentioned a 'water-filled truck.' I'd like the antithesis of a submarine with WWII-level technology. I don't know that steel and titanium would be practical.

What would a water-filled "landmarine" look like?

Assumptions include our typical mermaid and men, population, politics, industrial resources, and geography similar to Earth: I understand that these are all a dramatic stretch, to say the least, but this is worldbuilding.se They want dominance over the land. There are no air-breathing humans, and these merpeople cannot breathe air.

+1 to anyone who can find what the heck this thing would be called. Landrover? O-Boat? Are there any terms in literature for this?

• Well, sub-marine means below-water. So if you wanted an exact inverse... superterrestrial? Landrover sounds cool though. Or you could just call it a... tank? – IndigoFenix Feb 26 '17 at 6:12
• I like supmarine since sub/sup are opposites in familiar context. – JDługosz Feb 26 '17 at 8:18
• As for precedents in literature, see James Blish’s Surface Tension. I see the original 1952 magazine can be read online. – JDługosz Feb 26 '17 at 8:21
• I don't really feel that land bound vehicles would be the inverse of submarines; submarines can choose the distance from the surface. The machine you're desiring is really only the inverse of a modified kind of boat that sits so low in the water it needs to be sealed, which would fill the same roles as regular boats for humans (perhaps with added stealth). Encapsulated boats are not terribly spectacular, which makes land-boats not terribly spectacular. – StarWeaver Feb 26 '17 at 15:13
• Shouldn't they be called 'supterra' or 'superterra'? – RemarkLima Feb 27 '17 at 7:53

## I'm sorry to disappoint you but they wouldn't look like anything special.

Unlike crafts designed to operate underwater, these supermarines (yes, that is indeed what they are called, I will have my +1, thank you) would not need to contend with enormous pressures.

Further more these supermarines would not need any fancy mechanisms to keep the water sealed. Just a simple box filled with water would suffice. They wouldn't even need to periodically refill with oxygen. Just having some elongated tubes coming out from the top letting the air dissolve into the water would work. You could even open the top of the supermarine every once and a while.

So yeah, in short what you're looking for is a light tank-like vehicle with a retractable top, or weird tube things sticking out. Instead of a mounted gun (no human to shoot after all) just have an excavator hand on the front to collect resources with.

You know if you want more entertaining responses, you have to give us something fun to work with. What do you expect they would look like?

• Comments are not for extended discussion; this conversation has been moved to chat. – Monica Cellio Mar 1 '17 at 3:56

A water-filled tank would be both heavy and fragile.

Heavy : if you put it on wheels, it would require roads to be built first. I imagine that building roads on earth is as difficult for merpeople as it is for us to build under the sea (French Commandant Cousteau once did a trial without much success). Caterpillar tracks would be a better option.

Fragile : just drill a hole into it and it will leak and cause the death of the merpeople inside.

Solution : I would rather think of a scuba-like device allowing merpeople to carry water with them and have their gill kept wet enough with a built-in system to let O2 into the water and CO2 out of it.

Merpeople are however adapted to water, not to air. As a consequence, they do not only need water to breathe but also to move. That makes it more complicated than its human equivalent as it seems to me easier to take air underwater inside of a submarine (air is light and water carries the submarine and the air inside) than to take water outside to the surface.

Diverting a little bit form the original question and looking at its goal (domination), I would think at what we use submarines for : initially, to sink ships by carrying a bomb close enough and let it explode, causing damage to the hull. The bombs eventually became a torpedo. The torpedo eventually evolved into a missile to allow sea-air and sea-land attacks. But let's stick to the initial purpose of sinking ships. Ships are used to move from one shore to another. Let's thus imagine that merpeople from the mediterranean want to conquer the red sea. They would be interested in getting over and later controlling that stretch of land into which the Suez canal was dug. A first option would be to dig a canal similar to the one that de Lesseps made. Tough job for merpeople but at least it would solve the problem of breathing and would allow for the transportation of many people at the same time as opposed to vehicles.

In conclusion, my answer to your question would be : merman, don't try to build complicated vehicles, just dig a canal through narrow stretches of land in order to move faster from one sea to another.

• Why do we (humans) still use ships instead of building bridges across oceans? – Bergi Feb 26 '17 at 15:25
• @Bergi Because while we were able to dig the Suez Canal with the technology of 1859, we still find it challenging to build a bridge even over a relatively shallow strait of just a few miles, never mind between Europe or Africa and the Americas. Besides, a boat is just a wagon that floats, with propulsion suitable for the water; freight is easier to transport by boat than by road, and we didn't need to worry about how we could operate a ship without suffocating until we tried to make one travel under the surface. – David K Feb 27 '17 at 3:40
• In my opinion this is the only really good answer so far, because it addresses the problems with the assumptions of the question. – David K Feb 27 '17 at 3:41

Their technology would probably be vastly different: even if they have gained technology that relies on air since the industrial revolution, they will have a long history of building things differently. So they won’t have a steel box — steel will be reserved for the engine block and critical parts that require it. The vehicle will be adapted from what they use normally. Perhaps it will be made from bioengineered shell or some kind of concrete.

Rather than carry a truck filled with water, which is heavy and can fail catastrophically, they will use suits that keep the necessary breathing parts wet with fresh water, and cover any sensitive areas or use a personal drip system.

Getting around and bearing their weight will be a handicap when working dry. Maybe it will look like a wheelchair.

• The first thing is to act on land, the second is to master fire (only then you could start to use coal, oil, metals, engines, mass electricity production, batteries). I wonder how to ventilate the breathing water. The inflow of oxygen is about the same as for us, as is the egress of carbon dioxide. So a constant manual effort needed. Better to have chemical water refresher made of standard unobtainium. Seal-style leaps are sufficient for getting around. – kubanczyk Feb 28 '17 at 12:25
• @kubanczyk there are other questions about technological development of merfolk. – JDługosz Feb 28 '17 at 21:04
• Interesting idea. A wheelchair with a shower spout. – Mooing Duck Feb 28 '17 at 21:23

A fish tank on wheels. That would be the basic design.

The outside would be made of steel not glass course ( except for a few Windows that allow the crew to look outside). The inside will be a mini you water environment I would have everything you will need to survive a extended stay on land.

Including plants to provide food and oxygen to the water, a device on top made to stir up the surface of the water so that oxygen can also be taken from the air, a machine pumping air into the top of the submarine to supply first source of oxygen maybe even some sort of water filtration system prevent disease caused by waste buildup.

Certain sea creatures could also be brought to complete the environment serving as a sort of source of live food and as the way to clean up waste.

The sub will not be completely full of water though they would be at air pockets at the top provide contact with the air and exchange of oxygen from it.

The outside would look more like a sub mets tank. Mainly made of steel few windows lots of guns.

• how are they supposed to forge steel and glass? they live underwater. – Innovine Feb 26 '17 at 8:04
• @Innovine this was already solved in different Q&A on this site. – Mołot Feb 26 '17 at 8:53
• @Innovine where is the link – George McGinn Feb 26 '17 at 17:30
• I don't think the "supermarine" would have those plants or any mini-ecosystem, because that would be too small to do anything useful. Same as our submarines don't have plants taking care of oxygen need: you'd need way too much plants per person to make it useful. – hyde Feb 26 '17 at 17:45
• @hyde plant's is not the only source of oxygen just an additional one. – Bryan McClure Mar 1 '17 at 23:56

I would imagine that your water-bound creatures would have the type of airships that are used in the Iron Man and Captain American and S.H.I.E.L.D. Movies.

They only show an aircraft carrier, but instead of the atmosphere inside being a gas, it would be a liquid.

A sub as a simple cigar shape that is reminiscent of all the USO's (Unidentified Sea Objects) that people report either entering the ocean from the air or vise versa.

Your describe your world Earth-like, so it will have a gaseous atmosphere. That means for your submarine to fly, it will need wings. So it may look like a passenger aircraft, but it holds in water.

With WWII technology though, you don't have jet engines or fans big enough to move what would be a very heavy object around.

Take your German WWII submarine. It weights 871 tons. Let's say the interior space measures 250-feet long by 25-feet wide. The volume of a cylinder in cubic feet for a 250 by 25 foot submarine is 917,995.46

Your submarine will have to be small if your world has 1G like Earth. 1 cubic foot of water equal about 30 quarts. And 30 quarts of water weights about 63 lbs.

Besides the 871 tons that the sub weighs empty, add to it another 29 tons of water weight (used this to calculate volume) and if you keep to the size of a German U-Boat, it would weigh 900 tons. A lot of weight to get off the ground, never mind keeping it airborne.

I concentrated on airborne solution because any land-based craft with the added weight of water still, like a car or tanker, has to navigate the hazards of a battlefield, filled with craters, uneven terrain, and the energy required just to get something that heavy moving would probably be Saturn-V like in nature.

The mini subs of Japan that held two people and even the "Turtle" during the Revolutionary War will make more sense.

You see, if the year was 2525, then anti-gravity may have been perfected and size and weight not an issue. But with WWII technology, your mermen and women will have to operate smaller vehicles with the current world you descibed.

And I like the name Turtles. Even if they are small, just the weight of the water will make them move as fast as turtles.

• +1, They only have to do battle, on-land, between aquatic species. This is helpful, though. Would the Turtle need to be as strong as a submarine (since it is just holding water in at a very light pressure compared to holding water out at great depths). Hmm. Then again, it needs to be able to repel attacks. – Mikey Feb 26 '17 at 4:04
• Yes, and yes. However, I think once it is built to take some hits from small arms fire or glancing missiles, it should be strong enough to hold in the water, but I would have to do the area to pressure. The unknown factor is how deep down in the water do your creatures live. The further down the more pressure per square inch it exerts on objects in the water, and conversely, if you need the equivalent of 3 atmosphere inside, then your Turtle needs to be built as if it were going down to that depth as it has to hold higher pressures inside. – George McGinn Feb 26 '17 at 4:31
• A meter cube of water weights metric ton. that's simpler way to calculate masses :) – Mołot Feb 26 '17 at 8:56
• @Mołot Your're right, but us here in the Americas haven't yet adapted the metric system, which is a shame. I learned it for my cosmology studies, but when I write, I convert everything to what my country, and not the world understands. I need to stop doing that. – George McGinn Feb 26 '17 at 17:37
• @Mikey Do not underestimate the "very light pressure". The force is proportinal to the surface area of the tank, and unless you have a tank the size of bath tub, it's going to be substantial. Or to put it another way, if you have 2 cubic meters of water per crew member (still pretty claustrophobic), you already have 2 tons of mass before considering the vehicle itself. Now a truck/van capable of carrying 2 tons of cargo is probably going to weight 2 tons by itself, and then you might add 1 ton of equipment to keep the water clean and oxygenated. 5 ton truck, for one person. Did you want armor? – hyde Feb 26 '17 at 17:54

Not considering the supporting world, just the technical problem of a water-filled, mobile vehicle. There is a biological question tho': Do the merfolk need to be completely submerged? Do they breath with gill-like organs? Wouldn't it be sufficient to keep the gills wet? Many fish an survive for extended periods if their gills remain moist--gas exchange still works across the gill surfaces. If the vehicle were a refuge for rest and shelter, the merfolk could make forays across the terrain with a water-filled breathing apparatus. Let's call this vehicle a terrameris.

1. The terrameris vehicle doesn't need to be sealed--wouldn't want this anyway as a sealed container will present problems of gas exchange. So let's imagine an recumbent cylindrical container with a hatch or hatches along the top edge.

2. Water is very heavy, so minimizing the structural support mass will be important. The walls of the vehicle can be a clear polymer with external reinforcement. The cylinder has hoop-like ribs at regular intervals along its length. Rigid components could be ceramic or carbonate or manganese or a combination of all three.

3. Since the polymer can be flexible, this would allow segments of the cylinder to be articulated with large wheels or tracks supporting each segment. This will allow the vehicle to move across uneven terrain.

4. Power for moving the terrameris has to come from somewhere. You're going to need to imagine how the merfolk have developed motors, fabrication capabilities, etc. What are their fuel sources? Hydrogen fuel cells perhaps? If you split the water molecule, you get hydrogen, which can be re-oxidized (with O2 from the atmosphere) for power, producing water as a byproduct. So power comes from hydrogen cells, and the 'waste' water helps to maintain the internal environment of the terrameris. Perhaps each segment of the vehicle has a separate power train.

In summary, I see the terrameris vehicle as a giant caterpillar-like form with articulated segments that moves rather slowly, powered by hydrogen fuel cells, and propelled by a series of wheels or tracks. Each segment has a hatch for egress and gas exchange. Perhaps the segments can separate and rejoin as needed.

• A little hint for the future: Once you reach 50 reputation you acquire the privilege to comment on everything. This will allow you to comment on the question if you think something is unclear. In this case I think it's okay, but please keep in mind not to answer if you think something is unclear. The problem is: if you answer and the OP clears some of the questions you raise in a different way by editing his post he may invalidate your answer. Which is a bad thing we don't want on this site. But good start: Welcome to WorldBuilding! Have fun! If you have questions @username us or come to chat – Sec SE - clear Monica's name Feb 26 '17 at 16:24
• I don't think @Tubeworm's questions were clarifying details or suggesting that the question is unclear - I think they are rhetorical, to illustrate the answer. It's a good outside-the-box suggestion: perhaps a vehicle is not needed at all, but simply the merpeople's equivalent of breathing apparatus and diving gear. – Bekahland Feb 28 '17 at 14:04

The answers here all assume the merfolk can perform metalwork, which I think is unreasonable. So my answer attempts to provide a viable solution.

--They chew plastic--

Our oceans are filled with plastic waste. If the merfolk could chew the plastic and secrete a mild acetone, benzine or dichloromethane in their saliva (or other glands) they could soften certain plastics (like polyethyline) so it could bond with itself. It's possible that cyanoacrylate (superglue) works underwater too, enabling bonding with other plastics and rubber. Have the merfolk chew enough plastic into a paste and form the supermarine in a process like paper mache.

It might be possible for the merfolk to take plated steel from sunken ships, bend and deform it to shape, and seal the edges with their chewed plastic goop. Maybe they could use other excreted substances as a sealant, too.

To move around on the land requires a means of propulsion. Again, merfolk cannot fabricate metal parts and internal combustion engines, and even wheels on axles would be hard (and not commonly found on sunken ships either). A simple locomotion system would involve punting the vehicle around using long poles which they hang over the sides (think like a barge in venice) but this would be painfully difficult without decent wheels and all but the tiniest of vessels (minimum weight for a one person bathtub is going to be close to a half ton).

Given these locomotion restrictions, I think that the merfolk are going to be limited to a floating vessel which operates in water only. Essentially a floating bathtub driven with long poles and hooks, it would allow the merfolk to explore coastal water and move up a river estuary, possibly reaching a freshwater lake (I assume freshwater is poisonous to them, so this is an achievement for their explorers).

As saltwater won't float on freshwater, they will need air filled ballast tanks to keep afloat (alternatively, bottom crawling with the use of poles and hooks may also work, but I assume they want to see the surface).

• Cyanoacrylate can be obtained by tickling certain barnacle larvae and rubbing them on the parts to be joined. – A. I. Breveleri Feb 26 '17 at 9:53
• They were also called "typical mermaid" like, which depending on which version you believe, Disney or NatGeo, they have at least two arms, hands and 10 fingers, and either a fish-tail (Disney) or possibly two legs fused together by evolution. Either way, they have a brain our size and opposable thumbs. There are more metal and even diamonds (Carbon) in the oceans than on land or in the crust of dry land (worldoceanreview.com/en/wor-3-overview/mineral-resources) so they could have metal, and all the power they need for propulsion (2 Hydrogen atoms, 1 Oxygen, both raw forms of energy). – George McGinn Feb 26 '17 at 17:26
• @GeorgeMcGinn so they light underwater fires in their underwater smelters to forge their metal, is that it? – Innovine Feb 26 '17 at 22:46
• @Innovine Who said they had to build everything underwater? During WWII we had technology to use blow torches underwater to weld items together or cut chains holding mines. So one solution is that they have the same technology so they could use the technology in different ways. Or, they can put on their above water pressure suits and build the on land, they plan to use it above water. When we built the bridges in NYC (GW, Varrows, etc.) we had many divers welding and using fire and heat sources. It would be no different for the menfolk. – George McGinn Feb 27 '17 at 8:45
• Why would you suppose the oceans there are also filled with plastic waste? There are no humans there producing it. – Paŭlo Ebermann Feb 27 '17 at 9:32

You haven't adequately described your world. The first tool we had was our hands and we made hand tools. OK, mermaids can do that. The next was fire. A problem. So, what do we do about internal combustion, or worse, the Iron or Bronze age? Or glass? Or chemistry. I suspect development of a technological society under the sea would progress through biological means. Use of animals for energy, use of electric eels (say) as batteries. There's no reason that such a society wouldn't "invent" sailboats or dirigibles. The idea that our subs "occupy" the oceans is so laughable, that I won't say anything more about it. The merpeople have no need to "occupy" the land...how many undersea colonies are you aware of? They would no more want to live up here than we would down there. I'd suggest that sailing balloons is the way to go. They could only go as far as their feeble (unless you give them nuclear or petrochemical engines) energy sources would allow. But with enough caching along the way, it might be only a moderate problem - to organized (ie governmental) groups.

• It is exactly why I asked Worldbuilding for help. In clarification, I put assumptions that prior to this warfare, through whatever mechanism, the underwater nations are at WWII technology and resources, so I'm starting from that point. I reallllly like the ideas you started with, particularly the eels. This isn't about living on land, this is about domination of the land, much like our submarines were fighting for dominance of the sea, or protection of borders. – Mikey Feb 26 '17 at 1:52
• The 'Surfacemarine' would be known familiarly as, "The Fishbowl." – M.Mat Feb 26 '17 at 17:49
• +1 for the first part, but on the second, it's conceivable they may want to go on land for some reason, for exploration, or there may be valuable resources, it may provide a shortcut. e.g. imagine a tribe - or school, if you will - east of Central America wishes to get to the Pacific but there is no canal (or it's well-defended). Or they may know about fire but realise it can only be done on land. – colmde Feb 27 '17 at 9:46
• @colmde Fire means industrial revolution, so it's a big reason. They have a huge energy reserve with the coal and oil, but they need land presence to burn these, develop metalworking and produce engines. The engines would be generally used underwater (with snorkels). – kubanczyk Feb 28 '17 at 12:31
• The seamen wouldn't even have invented the wheel. They would drive regular submarines under water (just not airtight/watertight ones like ours). They would not even have invented the rolling pin (they cannot bake under water, and eating pizza under water appear to be a pretty ridiculous idea to me). – Klaws Mar 2 '17 at 9:00

I would take a more contrarian view; they would be very challenged to get onto land. The issue is that when we went undersea in 1941, we took batteries to power our life support systems. We recharge the batteries using diesel engines on the surface. The surface - AKA air - is everywhere. Our submarines were never more than 500' from air.

I think, then, that "a truck full of water" is a poorly thought-out answer. Where's the food? Where's the fresh (breathable) water? Where's the power supply? it isn't like IC engines (or pouring cast iron for them) is a natural thing for undersea dwellers.

Without thinking through this, posters are creating a Turtle that would be even less effective than the original.

I am assuming that like a WW2 submarine, the vessel must be able to be "out" for up to 6 weeks.

How will they keep their water breathable?

How will they dispose of waste? German subs in WW2 were supposed to be 'fragrant' by the time they returned to port. Imagine living in your own filth...

How does it move across land? Treads? Made of what? There are no roads, unless the Merfolk are making them.

How does it power itself? It is hard to imagine there are IC engines. What's the fuel?

How is the food stored? WW2 submarines hard a larder of dead things and cans. What will our Merfolk eat?

How fast is the vessel intended to travel?

How far from the sea is the vessel intended to journey?

How will the vessel communicate with 'home'?

The reason for the use of submarines in real life military operations is not to occupy the seas, but simply stealth. Yes, there is sonar, but this could be evaded in WW2 as well as today. One important concept is the thermocline, a boundary in the water with a pronounced difference in temperature between the water above and below it. This will reflect sound, so passive and active sonar are much less effective across this boundary.

How do the merman "see"? Do they use sonar, optical vision, smell? Would the rumbling noise of a land-based vessel prevent its use as a stealth weapon platform? Another option for a sneaky attack via land might be the building of canals or tunnels (below water level, or above water level with locks, like in rivers where ships need to traverse sudden changes in water levels). These might start out as civil trading routes, but could also be re-purposed in case of military conflicts (like autobahns).

Unmanned vessels (rockets, specifically) could be used both tactically as well as strategically. They can move faster than torpedoes, might be invisible to the mermen during most of their flight phase and an explosion in water is probably more harmful to sea life than it would be in air/on land to land life ("fishing with dynamite").

Back to the out-of-water submarine: I suspect that the lack of land-based, air breathing humans would mean that large areas of the land would either be covered by forests or be deserts. Forests might prevent the easy passage of any vehicles. Deserts might be an issue because of the large temperature ranges encountered there. If the vehicle carries enough water mass, it could possibly mitigate the effects of cold nights and hot days. I assume that the merman are poikilotherm, like fish. They cannot sweat in water, so they might be in real trouble when the water gets too warm, and swimming into colder water regions is not an option if you are caught in a land-based vehicle. The fun part is, of course, that this weakness might be of use in combat, where one side might try to heat up the enemy vessel by directing sunlight on it (via mirrors), or by using Napalm-like weaponry. This will be a very slow process, however. The "sunlight heat ray" option might or might not work for a larger land-based base, depending on whether attackers can move fast enough during the night (before they hibernate before they get too cold, or because the range of the heat ray is so large and the land-based vessels so slow that they will be sitting duck when the sun rises again. Obviously, attack vehicles could have a mirror surface, and equally obvious, their enemies could carry paint bomb to combat the mirror surface.

So, perhaps a dry ice reserve in the land-based vessels and a coal/oil/gas heater to cope with adverse temperatures? Or an "air condition" for the water?

Well, for short excursions onto the land, a rubber bubble filled with water might be sufficient, propelled by the merman walking inside. Dangerous, as the bubble might burst, the water might get too warm or the oxygen in the water might get used up before they can return to their natural environment. For longer excursions, a vessel might be a wheel, or rather a cylinder, filled with water. Unlike a real-life submarine, which has a form to minimize the drag of the relatively dense water, this cylinder would roll on the land and not care about a less than aerodynamic form. To allow for some sort of controlled movement, several cylinders (or wheels) could be connected. One famous example of a land-based vehicle with two connected "cylinder wheels" is the Flintstone car: https://en.wikipedia.org/wiki/The_Flintstones_(film)#/media/File:Flintstones_ver2.jpg

Just imagine that the "cylinder wheels" are large enough to carry machinery, merman, supplies and that no cavemen ride on top this thing.

Steering would still be a pain. However, real life military submarines aren't known for their cornering abilities, too.

Yes, there are already umpteen answers to this question. However, I will attempt to answer it in a way that none have yet done.

You say that your merfolk are at “WWII era warfare” — I take that to mean that they are at the second part of the Great War, and therefore are a parody of our own history, or some version of it.
Therefore, I shall suppose that the human nations and empires of that era were moved over to the eastern ocean: the Americas are in the Atlantic, Africa is in the south of India, Europe is off the coast of Japan and China — with the Isles conveniently separated from the main ocean by the ‘Nippon Insurmountain’?
Obviously it isn't a perfect conversion, because freshwater lakes do not have properties which equate them to islands for us, and I base my answer on the premise that these merfolk require salty water.

I'll say that they cannot control a heterostatic pressure of their bodily cells, and so are unable to venture from their home depth — which is what?
Are these merfolk bathic, occupying the deepest depths and possibly surviving off the sulfurous hot vents issuing forth from the cracks in the trenches?
Are they less extreme, and living at the transition from lit oceans to the dark depths? Feeding from the detrius which rains down from above, like mana from the heavens?
Or, are they shallow folk, farming the phytoplankton for nutrients?
Whatever the case may be, they are limited to it, and so it merely serves to provide them with larger expanses if they can occupy the seas above continental shelves.

Now, being as they are a parodical version of our history, you ask for a description of their Überlandkarts (U-boat, Unterseeboot).
Well, keep in mind that these people have probably already discovered a way to travel under, over, or through the massive obstacles of land. Why? Because these merfolk like to expand their limits. If nobody else has done it yet, then to do so is to reduce the competition and to allow previously stifled people a chance to thrive.

If they were shallow, it is quite simple. They've already begun colonizing the land with big umbilici which they've built against the rivers flowing into their oceans — remember that I suppose they require salty water. The same technology can be used to build bubbles on the land: waystations off the roads, where planktons can be cultivated.

Ah, yes, their technology. I will say that these people have technology which is much nearer their own biomechanical systems than anything else. Okay, so they aren't exactly like people, naked mediocre and extracting their living from the unwilling earth about them.

The deeper merfolk would be the ones who would require vessels to travel out of their stratum. I don't think digging would be an optimal method of expansion — though it certainly wouldn't be unattempted, especially with the folk huddling around the hot vents. Those ones are the most promising, so I'll go with them.

The hot vents, or hydrothermal vents (see to Wikipedia, NOAA, or PMEL), which exist on the ocean floor exude water heated to temperatures anywhere about the range of roughly 300 to 400°F, saturated with minerals, and with a basic pH. From my recent reading, many of the characteristic mineral chimneys which occur at these focused vents are mostly made of a copper–iron sulfide e.g. chalcopyrite on the insides — and zinc sulfides on the outside of the porous chimney walls.
Now, whether the merpeople are able to smelt any of the abundant ore or not relies on whether they are able to employ a means by which to separate the ionic metals from their companions to produce elementally pure metallic compounds.

There are two major ways by which to extract metal from ore. The first is chemical: chemically, whereby you introduce an ionic compound which causes the undesired components of the molten ore to either float or precipitate away, leaving the metallic elements alone; electrically or galvanically, whereby you use electric voltage to perform the separation, attracting the metal to the cathodes of your galvanic cell.
Well, anyways.

These merfolk, establishing their cities around the hot vents, would eventually learn that they could gather minerals from the distant brine pools. The brine is laden with alkali halide minerals, e.g. sodium chloride, bring them to the vents, and create voltaic batteries by separating the copper and zinc ions by their specific elemental gravities.
I've not worked out whether this is entirely possible, mind, but depending on the rigor of your world's scientific consistency, you may be able to simply say that it happens.

Suffice it to say that these merfolk would've already been building metalcraft in great smithies constructed over artificial extensions to the hydrothermal vents.

They would build vessels contain their own native pressures and allow them to venture upwards out of their depths, discovering the vast anti-depths above them. I figure that they would take comfort in the presence of a floor, much as we do, and so would eventually learn that it climbs upward.
They'd be establishing colonies all along the continental shelves — this is during the ages of 1600..1900.

Here, I must raise issue with the plausibility of them developing Überlandkarts, or even Überseeboots, which would venture to the surface of the oceans.
See, most of our submarine vessels didn't venture far from the ocean surface. The differences of pressure which they were built to withstand at even 1500 feet were in the ballpark of 660 psi absolute:
$$P_a = \rho h + 14.7\frac{\text{lb}}{\text{in}^2}$$
A very simplified version of the equation for the relationship between absolute pressure ($P_a$) in a liquid of a certain density ($\rho$) at a depth ($h$), and is valid for static pressures in a range of depths below Mean Sea Level where the water has negligible change occuring with its density.
Most of our submarines don't go so deep. Going to the benthic depths was not a routine activity during 1940.

Nowadays, we do built vessels capable of holding pressures in excess of 3000 psi, but those are built much differently than a submarine. Submarines have much more complex shapes and many more necessary seals to prevent leakage and to withstand buckling where the shape deviates from a perfect sphere.
Enclosing greater pressures on the inside is easier than keeping them out, yes, but there is yet a large differential between the pressure at 30,000 feet below Mean Sea Level and at MSL.

I don't expect that these merfolk would be straying on the land any time soon. You know why so few specimens of deep see creatures are adequately studied by us? Because they deteriorate when brought up to our comparatively low–pressure atmosphere, and must be kept in hyperbaric chambers to maintain their integrity.

I imagine it would look like a tank with a little bit larger window openings. Heavy construction would allow it to be pressurized and tank treads would allow it to travel without roads. They wouldn't want the sides completely open to the light because underwater the light is filtered down to a less intense level so they would want some shielding from the bright surface light. They would need some means to communicate but we have underwater microphones so that's no problem. It would have mechanical arms to manipulate objects outside of the vehicle.

Name suggestions: exaqua or interra
In Latin "ex aqua" means "out of water" and "in terra" means "on land" .

The Santini Box

A real-life surface transport for dolphins.

http://www.aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/1972/Aquatic_Mammals_1_1/Dudok2.pdf

Regardless of this specific implementation, the general approach for the surface transport of dolphins is to have a hammock (with cutouts for the fins) and a shower which keeps the dolphin from drying out.

The "traditional" depiction of mermaids and merman indicate that they can survive outside the water for short periods of time. If they evolved from lungfish, perhaps even longer (some African lungfish can survive for four years in dried mud, recycling their water, keeping their tail fin over their eyes to prevent these from drying out and breathing regular air).

Since, in real life, dolphins are not used in military surface operations, the actual vehicles are standard trucks or vans. For military surface use of dolphin, I presume something like an M113 armored personnel carrier could be used.

Note: I am aware that the OP specified that no humans live on the planet. The only conceivable way I can think of that the mer people would use names like "Santini Box" or "M113" would be some sort of contact with humans, possibly from off-world visitors, long range communication or reception of radio transmission, not necessarily specifically meant for them (like TV shows being transmitted to an other planet or solar system, or to a space vessel on the way to some solar system, to keep the crew entertained during their sub-lightspeed journey).

What about using normal land-based vehicles fitted for their possible lack of feet (driving with a joystick) and giving everyone their own life support suit like this guy in addition to a common one (water sprinklers or something like that)?

That way they are more efficient as a simple puncture on the hull could kill everybody. Since they are soldiers they are expected to be able to fight at closed quarters eventually.

But, if they got no way to move on land beside crawling (hello there, surgically split legs) they will be an easy group target, so less people per vehicle is better to minimize personnel losses during manouvers.

Maybe a lot of individual tanks (with internal life support system, in addition to the backup suit) similar to the one below. Or a modular big one.

• Where does your fishman picture come from? I know i seen it but i can't remember where? – Sefa Mar 2 '17 at 9:33
• @Sefa it should be the second hellboy movie (as noted in the alt text btw) – beppe9000 Mar 4 '17 at 1:34

If nobody has mentioned it yet, a fishtank on wheels is very little different from a car, and a car is not a submarine. At best you could say it corresponds to a boat in that it will ride on the surface of land instead of water. A submarine operates at any depth (within limits) underwater, so, by comparison a "supermarine" should operate at any depth (within limits) abovewater. This makes it more like an airship than a car. Without going into extensive analysis of underwater chemistry, or conducting chemical manufacture underwater, it is good to note that it would need to be "launched." Light gases come to mind, which work as well below as above the water. The depth issue applies, but since submarines are launched at sea level, so could supermarines--just in the opposite direction. A major difference is that water is heavier than air, so instead of being like a submarine and filled with air and a purification system for same, the supermarine may have a much more contained life support system filled with water due to its weight.

The vehicle itself wouldn't need special adaptations for the water-breathing merfolk. Instead, they could wear wet-suits with water-filled "re-breather" water tanks to keep their bodies at the appropriate pressure and wetness levels and keep sufficient oxygen in the water to keep their gills happy. These might look like deep-sea diving rigs (see link).

They will have a bigger issue with making the vehicle itself. Propulsion systems that work under water are much less effective on land. They might have better luck stealing land vehicles, unless they have pretty solid R&D division.

Amphibious out-of-water submarine would look like a submarine fused with hoovercraft where propellers may act as pushing fans on ground and air in float tanks could be used in bellow action.

• Indeed an hoovercraft does what turbines do – beppe9000 Feb 28 '17 at 12:11
• Hoovercraft? Named for the vacuum cleaner brand or the U.S. president? – JDługosz Apr 18 '17 at 4:14