In my epic fantasy world set in the 1450's there are underground tunnels that literally go across from under the ocean for 700 miles or so. So here are my questions:

  • How long would it take to dig through this tunnels which would be made from rock and minerals.
  • How many men would you need?
  • Is there anything else I should take into consideration?

This is all for my worlds history, I know it's unrealistic to make this happen but I'm making an epic fantasy and as long as I know the numbers I can create a cool story about why/how this was made.

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    $\begingroup$ Weight estimates for structural supports $\endgroup$
    – nzaman
    Commented Dec 1, 2018 at 10:50
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    $\begingroup$ A tunnel under the ocean? That must be deeper than the ocean, at least something like 500m, likely much more. And solidly isolated from the sea water. Should your excavators ever hit a patch of sand, clay, or whatever other loose material that's connected to the ocean, well, that's it for the excavators and the tunnel they've dug so far... With that level of technologies it's virtually impossible to finish such a tunnel. Today, we could do it, but only because we can analyze the ground beforehand via seismic measurements - and it'll still be dangerous and likely to fail entirely. $\endgroup$ Commented Dec 1, 2018 at 11:02
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    $\begingroup$ A 700 mile tunnel is going to need a lot of ventilation, otherwise the miners and anyone using it will suffocate. You might look into the ventilation system for the Channel Tunnel, which is less than 5% as long as your proposed under-ocean tunnel. $\endgroup$
    – jamesqf
    Commented Dec 1, 2018 at 19:52
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    $\begingroup$ Why has this question been flagged as Too Story based (TSB)? This is nonsense. This is about an worldbuilding elements. Namely, sub-ocean underground in a 16th century world. Doesn't matter if it's unrealistic in the quotidian world. It is for a fantasy. I recommend whoever VTCed this as TSB to reread the link to the TSB criterai & gain understanding of them. $\endgroup$
    – a4android
    Commented Dec 2, 2018 at 2:43
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    $\begingroup$ Frankly, in 1450 a 700 METER continuous tunnel would be one of the Wonders of the World. Even in 1840, with the use of much more modern technology and the might of the city of London pushing for results, a 1300 foot (<400m) tunnel under the Thames river took 18 years and the lives of several workers... That's for 1/4 mile. You want 700 miles, and older tech? $\endgroup$
    – PcMan
    Commented May 23, 2021 at 14:54

11 Answers 11


I'll see if I can find a rate at which manual workers can dig a land tunnel but, in your case, it will take forever because ...


Egyptian and Roman mines were worked to depths of approximately 200 meters. (Mahtab and Grasso) By the 6th century B.C., it has been estimated that the advance rate of a hand-worked tunnel in hard rock was perhaps 9 meters per year.


EDIT Borrowing from the rough calculation by A random person no.0 and multiplying it by 1000 to generously correct the speed, we get about 300,000 years.

Apart from speed, Flooding will be your chief enemy. A quick Google search turned up this link

The Abandoned Victorian Passages of the First Channel Tunnel

enter image description here

There had been numerous proposals for a tunnel under the channel throughout the 19th Century including one by Napoleon, but the first serious attempt to build a tunnel came with an Act of Parliament in 1875 authorising the Channel Tunnel Company Ltd. to start preliminary trials. This was an Anglo French project with a simultaneous Act of Parliament in France. By 1877 several shafts had been sunk to a depth of 330 feet at Sangatte in France but initial work carried out at St. Margaret's Bay, to the east of Dover had to be abandoned due to flooding. http://www.subbrit.org.uk/sb-sites/sites/c/channel_tunnel_1880_attempt/index.shtml

Some random links (don't forget to do some basic research before asking a question ;-)




  • $\begingroup$ Wasn't aware of that early channel tunnel trial - thanks. $\endgroup$ Commented Dec 1, 2018 at 23:16
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    $\begingroup$ Just to add that you can't dig through a mid-oceanic ridge because, under the sea bed, it will be lava. $\endgroup$ Commented May 23, 2021 at 13:40
  • $\begingroup$ Wait wouldn't it be 34,000 years? $\endgroup$ Commented Jun 23, 2022 at 17:07

set in the 1450's there are underground tunnels that literally go across from under the ocean for 700 miles or so.

Brunel's famous Box Tunnel was 1.83 miles in length and took 2.5 years to build (which involved explosives your 1450 miners would not have). Extrapolating from that it would take something on the order of 956 years to build your tunnel.

Note this ignore some extraordinary challenges that the Box Tunnel did not have, not least of which are transport for material excavated, miners and the heat, pressure and humidity at that depth combined with the more primitive technology and the "minor" details of breathing and flooding. Anything less than 1500 years seems very unlikely.

Note that no matter how you do this you are limited by the size of work face. You can't add more people to this and get it to happen more quickly.

In the case of the Box Tunnel they had vertical shafts used to provide ventilation and easier means of extracting material and bringing in supplies and men. This is not quite impossible with an undersea tunnel, but is almost certainly beyond the capability of the engineering of the 1450's (although if you want this tunnel for use in the 1450's, you'd have to start building it about 1500-2000 years earlier somewhere in the BC period on our calendar !).

Brunel also had to undertake a study of the geology of the tunnel and that itself took a year, so the same process for your undersea tunnel would be even more complex and increase the time to dig from about 2.5 years per 1.83 miles to at least 4 years per 1.83 miles. So something like 1530 years !

How long would it take to dig through this tunnels which would be made from rock and minerals. - How many men would you need?

Best estimate about 1500 to 2000 years !

Using an average of about 1000 to 4000 men dying at the rate of about 50 to 250+ per year - rock is dangerous, so is any form of excavation.

You also need the resources to make the vast quantities of materials to make the tunnel.

So to be in operation in 1450 your tunnel would need to have begun being built in what on our calendar would be 500 BC to 0 BC.

If you start it in 1450 it might be ready in 3000 ! It's actually quicker to wait for the technology to travel faster on and above the surface to develop.

With the same technology level and considerably less resources you'd build and maintain a vast fleet of ships capable of continuous transport of people immediately across the ocean and without being tied to a single route.

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    $\begingroup$ If the tunnel is in the open ocean (rather than continental shelf, like the Channel Tunnel), then 15th century technology wouldn't even be able to determine the depth of the ocean. (E.g. Shakespeare's "But it cannot be sounded; my affection hath an unknown bottom, like the Bay of Portugal".) Even something as prominent as the Mid-Atlantic Ridge wasn't discovered until the 1872, and its extent wasn't really known until the invention of sonar. $\endgroup$
    – jamesqf
    Commented Dec 2, 2018 at 4:47

The only solution I can come up with is a geological one. The tunnels were the result of underground rivers flowing over millions of years. Then the sea-level rose and covered them. The explorers find one entrance (in a diving bell?) and apart from a certain amount of silt near the entrance they find a huge system of underwater tunnels. Of course they are full of water so they must find a way of sealing any leaks and then draining hundreds of miles of tunnels. Probably harder than removing rocks because until the leaks were fixed the tunnels would fill up again.

It's not very convincing but I thought I'd include it in case it inspired the OP or someone else to make it more plausible.

A geologist might be able to say whether flows of lava overlaid with silt which then turned into rock could create a soft rock to dig through with a tough outer shell. I'm no geologist so I don't know.

  • $\begingroup$ I "dig" this idea, specially considering that it is a fantasy world. Another planet with its own geological peculiarities? Maybe the crust is full of old lava tubes? Throw in some religious kingdom bent on maintaining the tunnels and make nice descriptions showing how hard it is for the miracle tunnel to be possible (and how all things are affected by its existence) and I´ll suspend my disbelief for as long as the created world and background is fun and interesting. $\endgroup$
    – mcbecker
    Commented Jun 23, 2022 at 18:10

As several answers say, this is impossible. One solution is to (very early in the book) attribute the construction to a previous advanced civilization with mystery tech capable of this feat.

A more plausible solution is not a tunnel, but a kind of rail road. The Vikings in 800 AD had crucible steel, in some cases superior to even modern steel. They only used it for top-class Ulfbehrt swords, but imagine if they had used it for machine and other purposes sometime in the following 650 years. In particular, they might have figured out how to lay a big steel cable and anchor it to the sea floor (Lead lasts centuries under sea water, especially if lacquered first). I'd put a thick coat of lacquer on the cable, too. So an ox-powered "carriage" (a big submarine basically) could drag itself along this cable and cross the ocean.

I think you'd still be better off with ships, or a series of giant floating circular rafts anchored to each other, a hundred yards wide. Even if broken by hurricanes, that is a highway that could be repaired.

  • $\begingroup$ I like the previous advanced civilization explanation. $\endgroup$
    – Willk
    Commented Dec 3, 2018 at 1:48

You just need to "make it so" and set a time and ignore the issue of believability

At this site I found the following reference:

I've had some experience with drilling hard rock on Canadian Shield granite with a Gardner Denver 800 cfm (200 hp) compressor and drill rig using a 3 inch bit. We would generally be able to drill 300 feet in 8 hours using percussion. Percussion is somewhat like using a pick.

200 hp gives us 300 feet x 12 inches x 3.14 x 3 inches squared / 4 = 25,434 cu. inches chewed up in eight hours. ... Now I know that an athlete can put out 0.25 hp for short durations. At this point I'll assume that you can put out something somewhat less, lets say 0.1 horsepower for 6 hours.

Therefore I'm figuring you might chip out 9 to 10 cubic inches.

Now, you tell us nothing about the tunnel other than the length. So, some assumptions:

  • We need to be 100% in bedrock because there is no tech in the 1450s short of magic that can hold up an ocean.

  • The tunnel would need to be enormous to carry air through that length of tunnel (it's probably still unrealistic, but let's roll with it). Let's say at least 30' in diameter.

  • The average depth of the ocean is 12,100 feet with depths to 36,200 feet. Let's just assume the average and assume you're 100 feet into bedrock to guarantee the ceiling stays up (I'm going to simply assume nothing leaks. That's a falshood so magnificent that the devil laughs and angels weep, but we'll assume it). So, you need to dig down from the beach, 12,230 feet, then across 700 miles, then up again another 12,230 feet.

At 10 cubic inches per person per day

You could have about, oh, 20 people actually working the rock face with ladders. Even if you're working from both sides at once (a miraculous feat of engineering in the 1450s), you're hauling the debris back 350 miles and lifting it 12,000+ feet. (You know you'll be creating mountains with the tailings piles, right?)

So, 20*10 = 200 cubic inches a day (we'll ignore the cottage industry keeping the wheel barrows and picks in supply) or 0.1157 cubic feet a day. The volume of a cylinder is easily found online, so you need to remove 2629838205.89 cubic feet.

22,729,802,989.58 days | 62,273,432.85 years

Read that slowly... 62.3 million years

OK, that was dramatic, but the rating was for only 6 hours of work. So we need to divide it by 4.

15.6 million years.

OK, ignore the engineering miracle, we're working from both sides...

7.8 million years.

How many people?

I know some modern miners who can work all day — but they're not swinging picks, they're handling drills. You'll be swaping out guys every 2 hours. Maybe they can swap back in. Let's say you can do this 3 times in a day. That's 4 teams. 40 people.

But at the end you need to move tailings 350 miles (at best, wo and we're breaking rock 24/7. Your average wheelbarrow can carry about 4,000 cubic inches (this is a guess-an-average based on looking at a few wheel barrows online). So you're moving one wheel barrow every fortnight — but that guy won't be back for a while.

Your average walking speed is 3.1 miles per hour. Our workers are in great shape, but they're pushing a wheel barrow. Let's say 3 mph. 350/3 = 116.67 hours or a round number of 5 days to walk out.

Without sleep...

At 10 hours per day we're talking 12 days to get out. That's almost as unbelievable as the 7.8 million year number, but let's roll with it.

This guy can't quite get back in time for the next load, so you need 2 dudes working the wheel barrows. Someone to cook at every 1/2 day point (24 dudes), someone to move water and food (2 more dudes), we're feeding the 40-person crew, 8 more dudes, and who knows how many are supporting on the outside. I'm going to ignore clothes, repair, etc. (I'm ignoring an unbelievable number of details... weeping angels...)

Oh, let's round it up to 100. On each side. So 200 dudes.

The problem is you can't get more on the rock face, and that's the tall pole in the tent.


7.8 million years...

Start at 15, end at 55: 200 dudes for 40 years. 195,000 generations...

39 million people. Excluding the babies, wives, old folks, and everyone supporting the 200 dudes.

Is this at all believable?


Is this at all useful?

Nope. Your tunnels were built by magic or using equipment a whole lot more powerful than picks.

Before you jump to explosives...

You need to understand air blasts. I used to live in Idaho's Silver Valley, where there are many mines and a lot of graves holding the bodies of miners killed via air blasts. You're inside an entirely inflexible tunnel. The force of blasting must go somewhere. Where it goes is through the heads of your miners all the way back to the tunnel entrance.

This means vacating the tunnels whenever you blast. but blasting will only convert cubic inches to cubic feet. It's not going to bring the time down to centuries. You'd be lucky if it brought the time down to 100,000 years.

And you'd no longer be able to do it with just 200 people (39 million over the span). You'd need thousands. (Probably still 39 million, just over the shorter span.)

For one 700 mile long tunnel

  • $\begingroup$ I think your estimate for the mining speed seems extremely conservative. It's still going to take a long time, but I'd guess at centuries not multi-millenia. $\endgroup$ Commented May 11, 2021 at 13:22
  • $\begingroup$ @Pureferret, it wasn't my estimate. I cited the source for the estimate. I'd be delighted to update the answer with a better estimate. Can you cite a source? (Note that unless you're a mine engineer, your guess isn't valid. I'm not, which is why I found a source for the estimate.) $\endgroup$
    – JBH
    Commented May 11, 2021 at 16:41
  • $\begingroup$ I'm working on an answer, and I'm planning on taking from multiple historic sources. My issues with the source you found is that although they're an expert in percussive drilling, that doesn't mean they know that a person puts in that less HP, or that it's scales linearly. $\endgroup$ Commented May 11, 2021 at 17:34
  • $\begingroup$ if you try to ig from both sides you run into the problem of digging precision that is staying straights for miles undergrounds. if you are off by a few microns over hundreds of miles your tunnels will pass each other by without meeting. $\endgroup$
    – John
    Commented Jun 23, 2022 at 17:46
  • $\begingroup$ @John Um... did you miss "a miraculous feat of engineering" and "ignore the engineering miracle." I addressed your issue. I just didn't spend any time on it (I felt it was unnecessary compared to everything else). $\endgroup$
    – JBH
    Commented Jun 24, 2022 at 9:19

Instead of building a continuous 700 mile undersea tunnel, just build the parts of the tunnel you need. So if you have a car full of people and goods that is being transported under the sea, you just need a chunk of tunnel bigger than the car, which moves with the car. In other words, a submarine. You can get the logistical benefits (and strategic risks) of a tunnel by building a cable system to pull the submarines along the sea bottom.

So what you get is a cross between a cable-car and a submarine. Choosing strong corrosion-resistant materials is left as an exercise for the author. Gutta-percha will probably be used somewhere.

The project will have most of the difficulties of laying undersea cables, building long railroads, and building submarines. The biggest challenge will be building a suitable submarine. In our world, this took about 140 years, from the American Revolution to World War I.

As with regular railroads, undersea cables, and telescopes, it is better to start small. "It is cheaper and faster to build a 4" telescope, and then build a 6" telescope, than to start by building a 6" telescope." Similarly, your heroes can start by building a cable car under a lake, and work their way up to longer routes.

In our world, it took 40 years to go from trial horse-powered railroads to starting a transcontinental railroad. The transcontinental railroad took another 6 years to build.

Neal Stephenson wrote a fantastic non-fiction article on how to lay an undersea cable:

Mother Earth, Mother Board

The hacker tourist ventures forth across the wide and wondrous meatspace of three continents, chronicling the laying of the longest wire on Earth.

In which the hacker tourist ventures forth across the wide and wondrous meatspace of three continents, acquainting himself with the customs and dialects of the exotic Manhole Villagers of Thailand, the U-Turn Tunnelers of the Nile Delta, the Cable Nomads of Lan tao Island, the Slack Control Wizards of Chelmsford, the Subterranean Ex-Telegraphers of Cornwall, and other previously unknown and unchronicled folk; also, biographical sketches of the two long-dead Supreme Ninja Hacker Mage Lords of global telecommunications, and other material pertaining to the business and technology of Undersea Fiber-Optic Cables, as well as an account of the laying of the longest wire on Earth, which should not be without interest to the readers of WIRED.

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    $\begingroup$ why the heck would you build a submarine, when there are perfectly fine boats, which even have access to breathable air all the time? $\endgroup$
    – Burki
    Commented Jun 24, 2022 at 13:16
  • $\begingroup$ @Burki — Boats can be seen, which makes them much easier for other boats to interdict, destroy, or seize. $\endgroup$
    – Jasper
    Commented Jun 24, 2022 at 16:50

The level of magic you need to make this work mean you can say it was created in minutes and no one can really argue.


  1. continent to continent under the ocean means you are crossing either a subduction zone which means major fault lines and a trench or through a spreading center in which case you are digging directly through a volcano and magma. We can't even come close to doing either today.

  2. you tunnel will flood, massive rock is not waterproof, you need to be actively pumping the water out of your tunnel. Worse under oceanic pressures any water coming in means your walls start eroding and failing immediately, so you need magic waterproofing.

  3. you also need active ventilation, otherwise anyone in the tunnel will suffocate, miles of dead air space means oxygen replacement is minimal. You also need this to keep people in the tunnel from being cooked alive as the rock will be too hot for human occupation. rock gets hotter as to you get deeper, your tunnel is deep enough that the rock is hotter than humans can withstand around 150 F at minimum, and that is not even near the volcano you might be cutting through.

If you have the magic to do all that, then just instantly creating the tunnel is just as believable.

  • $\begingroup$ The subduction zones are no joke, but you also couldn't get past the mid-Atlantic ridge. That's a wall of magma the entire north/south length of the Atlantic ocean. There is no crust along that boundary, not even at the surface where it meets the ocean. So, yea, magic. $\endgroup$ Commented Jun 24, 2022 at 0:37

Warning: this is based on opinion and should not be taken seriously and I'm being sarcastic

So your miners are zombies, don't need food and rest. Let's assume that they can dig 100 meters a day, overexerting their bodies 24/7 (it's impossible) and remove the problems of logistic (bringing extra tools and support structure, materials, etc). They would take around 34 years to dig the tunnels (730 MILES long) and now, you have an unsupported tunnel ready to collapse any time. Now that's on the optimistic side of things, but feel free to pick any number from 34 or above because on the pessimistic side of things it will take forever to make it.

On realistic human digging speed (straight down) at that time a thousand years is the fastest, but support structures aren't included and you would have entered the modern age and be able to use machines to do it

About the number of men you need, it's 0 because it's zombies. (or around 1.25 million zombie workers and an infinite amount of planners (it's impossible) or 1 genius character for plot's sake)

edit: I've just just read your other questions, looks like you are going to move your animals using this tunnel?

  • $\begingroup$ Even though you were somewhat generous with the speed (see my answer for a more realistic speed for humans - in solid rock it's more like 10 m per year). I think the rest of your analysis is good. If you don't mind I'll multiply your final result by 1000 (to allow for them being zombies). $\endgroup$ Commented Dec 1, 2018 at 12:23
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    $\begingroup$ lol yeh that was the plan a few days ago, but it's just not going to work though, I'm going to completely scratch that idea out. Even if I create a whole history based on why these zombie were trying to dig their way onto the other continent millions and millions of years ago.. I feel like it'll sound dumb. I appreacte your help tho! great answer :) $\endgroup$
    – Beginner
    Commented Dec 1, 2018 at 12:42

Ok, came here looking for general ancient tunneling speed so here are somewhat unrealistic estimates.

In AD 41 the Romans used some 30,000 men for 10 years to push a 3.5-mile (6-kilometre) tunnel to drain Lacus Fucinus. They worked from shafts 120 feet apart and up to 400 feet deep

At this rate, traveling .35 miles a year gives you an entirely unrealistic estimate of 2000 years. However, with the zombie concept, it may be feasible... Maybe.



The key here is that we're talking about fantasy, so you have numerous possibilities for achieving this that aren't available in our world.

  • 5th level D&D spell "Transmute Rock": With this spell, you could convert the rock in front of you into a more pliable material like mud, and have workers just dig it out instead of having to use explosives. You'll still need mining cars to move it out of the tunnel, of course.

  • Earth elementals: Seriously, these guys don't even have to breathe, and I can't imagine a high-end mining operation that wouldn't make use of them. In this case, maybe the elementals would bond with the rock and draw it out to an area where workers could load it into carts.

  • Digging monsters: Some of these are overlapping with the earth elementals, but they might have super-sharp claws, and just need to be pointed in the right direction.

  • Scrying and elemental scouts: You won't find solutional caves under the ocean, but natural cooled magma tubes might exist for long stretches of the distance. This would also help you avoid pockets of explosive gas, areas of weak structure, pockets of pressurized water, etc, etc.

Generally speaking, if a fantasy race is going to do something like this, it's because they have some capability that makes it much easier than it would be for us. Otherwise they just wouldn't even start.


Is there anything else I should take into consideration?

That all of your men are dead. So you do not need to pay their salaries. Profit!

People consume molecular oxygen (O2) to live[citation needed]. The only natural process that we know produces molecular oxygen1 is the plants photosynthesis, that requires solar light to happen.

Places without plants get their oxygen through ventilation with the surface. And here is the catch, ventilation only works in the immediate vecinity of the opening, and air deep inside a cave will renovate with oxygenated air very slowly. Even in modest, existing holes (mines) and natural coves, if they are deep enough you will need either to carry your oxygen with you or to increase ventilation with pumps.

Obviously, ventilating a 700 km tunnel is impossible, and to pass through it takes so much time that you just cannot carry enough oxygen bottles with you.

Additionally, dependending of the geology your cave could very well be filled with poisonous gases like carbon monoxide or sulfurous gases.

If you handwave the air part, the problem repeats itself with the supply of anyone going through them. You will need to provide food and water to people passing through the zone, and a 700 km journey will need so many supplies as to consider it impossible (you can compare to Antartic expeditions).

1Atomic (O) and molecular oxygen(O2) are oxidizers, which means that the has a strong tendency to naturally bound to other elements.

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    $\begingroup$ Clearly you did not read the part where I said that I know it's unrealistic. Now let's say My Miners don't need photosynthesis because they are a different species. They don't breath as they're pretty much like dead zombies. And I am in no shortage of supply for them. You didn't answer a single one of my questions. $\endgroup$
    – Beginner
    Commented Dec 1, 2018 at 11:05
  • $\begingroup$ If it is unrealistic, then you need -1 men. If you are talking about "zombies", explain it in the question (although then they can just walk on the ocean floor, don't you think). And then do not use the science-based tag. $\endgroup$
    – SJuan76
    Commented Dec 1, 2018 at 11:07

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