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For as long as recorded history, the Earth has had two moons - religions were based around the interactions of these two bodies in the heavens, they were worshipped, feared and romanticised.

Then at the birth of astronomy in the 17th century, it was discovered that the smaller of the two bodies was in fact obviously artificial, with hatches, windows, tunnels, structures etc etc.

The first manned space missions in the 1950s and 1960s targeted this structure rather than the moon (which was largely forgotten as a goal) and resulted in multiple countries landing at various points on it.

Assume an accelerated rate of technological advance into space flight, with such technologies such as space planes, NERVA engines, Orion spacecraft et al all being followed through to a usable conclusion, but not more than 5,000 people being put into orbit between the early baby steps of the 1950s until the current date (circa 2050).

Also assume that the structure is not just surface shallow, it is not hollow, nor is it solid but rather its entire volume is a structure, how large would this structure have to be to still be largely unexplored?

Edit: In response to queries for further parameters.

  1. Assume that the structure is largely operational, with human compatible environmental systems, power, lighting, mass transit, gravity etc

  2. Assume that these systems are not necessarily free to use - doors may be locked (eg secure areas, personal accommodations etc), mass transit systems may need a form of interaction not immediately obvious (eg payment), there may be areas of the structure which are exposed to vacuum or irradiated, there may be damage from meteorite strikes.

  3. Assume a political climate similar to our own, on a similar time line - there was a cold war which ended in 1990 for much the same reasons.

  4. Assume motivations both compatible with a cold war, and also a general public desire for information - for example what does the structure mean, who built it, when was it built, what does it mean for us as a species, but equally the desire to retrieve advanced technologies before any opposing country or faction, to achieve the upper hand before someone else does.

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    $\begingroup$ Fermi estimate, assuming that most of the volume is "uninteresting" enough to not warrant close inspection: 1 person scans about 1 cubic meter per second. 5000 people x 1 cubic meter per second x 100 years = 15,800 cubic kilometers (equal to a sphere 20 miles in diameter). So, unimaginably huge in terms of a structure, but peanuts compared to the Moon. $\endgroup$ – 2012rcampion Apr 15 '16 at 13:46
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    $\begingroup$ Given the cost of spaceflight, even with better tech, if nothing new and exciting was found after 10-15 years, there might be a large public outcry to stop spending money on it. Leaving only a small outpost to maintain possession. $\endgroup$ – Mr. Smythe Apr 15 '16 at 15:04
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    $\begingroup$ Im pretty sure that they would have sent a ton of people over there in the hope to find something useful. You cant drop a thing like that in our same timeline and assume no changes in it. The maximum of the efforts would have put to squeeze all that was possible, even more because of the cold war. As for the masses the need to prevail on the other block and religion and meaning of life and origins etc they wouldnt say a word against spending money to explore it. Everyone and their dog would have wanted to go there and discover and take possession of something. $\endgroup$ – Erik vanDoren Apr 15 '16 at 15:29
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    $\begingroup$ Why send bulky people who need bulky food and oxygen and water? Why not send robots like we usually do? In Earth orbit it doesn't even have to be autonomous. $\endgroup$ – Schwern Apr 15 '16 at 17:08
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    $\begingroup$ Size doesn't matter, we've been looking the wrong way for inspiration. $\endgroup$ – Schwern Apr 15 '16 at 19:36
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This depends on a very large number of factors, like how hazardous the structure is and how large the tunnels are.

If they're too small for a man in a space suit to fit through then it could take a very very very long time.

If it's laid out in a neat grid it will take less time. If it's a rabbit warren then more.

If the structure isn't active and isn't trapped and isn't otherwise actively trying to kill explorers etc then I'd compare it to cave diving.

They need to carry their own air, they can't go very fast, it's dark, they're in bulky suits, they don't know what's around the next bend. So lets go with cave diving.

Cave divers can cover around 15 meters/minute when cave diving. Less if it's tight. Lets go with that.

Lets assume living spaces similar to the ISS.

So we've got 5000 people who've spent time mapping this. Lets assume that each of them has averaged the same as a tour on the ISS, 6 months each over the course of 100 years.

5000/100 = 50 people per year or 25 doing 6 months shifts.

so at any given time call it 25 people resident in stations attached to the structure, perhaps 5 stations similar to the ISS with 4-6 crew each.

They're unlikely to spend every waking moment exploring, they need to spend a large fraction of their time maintaining the systems that they need to live or doing work in the explored sections.

The longest space walk in history was eight hours and 56 minutes.

Lets assume they each spend, on average 3 working days per week purely on exploration for 9 hours solid. More and they're more likely to make mistakes and on space walks mistakes are likely to mean death.

Of those 9 hours they can't spend the whole time in new passages. They're going to need to spend time traveling through explored regions to reach new areas and time traveling back.

Lets assume 3 hours out, 3 hours exploring "virgin" terrain and 3 hours getting back.

They'd probably move their bases to new regions once the trips took too long but I'm probably making pretty generous assumptions.

Finally we can make some guesstimates.

15 meters per minute * 180 minutes per day * 3 days per week * 25 people * 52 weeks * 100 years

1,053,000,000 meters or a little over a million KM of tunnels.

That sounds like a lot but for comparison at 2,500km, the Odessa Catacombs are probably the largest network of man-made tunnels on earth.

I can't get exact numbers for the 2D size of the Odessa Catacombs but from some googling they seem to be mostly under an area with about 70 KM diameter.

So if your station was a rough sphere with a diameter something along the lines of 100KM with a winding maze of tunnels roughly as dense as the Odessa Catacombs throughout the 3D structure then we could expect it to have something like 5 million miles of tunnels leaving most of it unexplored even after 100 years.

Specialized tech for aiding the exploration would speed things up massively but then they're unlikely to have as many people up there in the 1960's as later. If you want to deal with that then make the structure mildly hostile to small drones and similar tech unaccompanied by humans.

to make this economic they're going to need to be finding something of worth. This would be a project dwarfing the moon landings in terms of cost. They'd need to find something equivalent to room temperature superconductors or working fusion generators every few or so at least.

EDIT: Some of the edits to the OP contradict my answer since I was assuming cold, dark, dead tunnels navigated in space suits.

However I only considered tunnels, not any of the rooms connected to them so it may still take about as long to properly explore the space. Also live technology allows for more interesting problems like "staff only" areas which attempt to expel our explorers since the system does not consider them staff etc.

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  • $\begingroup$ You are not taking into account technological development over the 100 years 1950 ~2050. As OP states, space tech advanced way faster than ours. $\endgroup$ – Mindwin Apr 15 '16 at 13:44
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    $\begingroup$ Ya, that's why I included the note about perhaps wanting to make the structure slightly hostile to unmanned drones in some way. It's still non-trivial. If drone tech advanced fast then some mission might just release a hundred million midge-drones spread out in a cloud and map the whole structure. One way to deal with that might be to specify that there are occasional doors or barriers that need to be opened with a lot of strength or a little explosives (non-trivial for small drones) $\endgroup$ – Murphy Apr 15 '16 at 14:16
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    $\begingroup$ @Mindwin I wouldn't say that the tech in this instance advanced faster, there was just more political will to finish off promising projects such as those mentioned - rather than the political will being withdrawn in the latter stages of Apollo (as a result of beating the Soviet Union, which was the real goal there), rather it ramped up because there was still a massive goal and immense potential of rewards awaiting. $\endgroup$ – Moo Apr 15 '16 at 14:58
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    $\begingroup$ "to make this economic they're going to need to be finding something of worth" Something of that scale and been there that long would have literally everything of worth $\endgroup$ – Erik vanDoren Apr 15 '16 at 15:35
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    $\begingroup$ @Mindwin so adding clarifying statements is wasting peoples time now? My point was that we could be well ahead of where we are in real terms right now if the political aspect had not killed a lot of promising technology in its infancy. We had the technology, we chose not to use it in real life - in my fictional scenario, we simply chose to use it. Thanks for contributing, but feel free to not contribute any more if that's the way you feel about it. $\endgroup$ – Moo Apr 15 '16 at 16:00
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UPDATE I have it, but it's not about size, and we've been looking the wrong way. Size doesn't matter, and we need to look down, not up, for examples of unexplored places.

If there is sufficient funding, it would have to be absurdly large and absurdly laid out. If there isn't sufficient funding, it has too much economic, military, and philosophical value to remain unexplored.

No, instead it has to be not too large, but too dangerous.


First, why size can't save you.

You say there's air, power, lighting, mass transit, even gravity (?!). Setting up a permanent, manned colony on the station would be far, far easier than setting one up on the Moon. You still need to ship humans, food, water, and their equipment but not air and environmental systems. There's no issue of bone degradation due to lack of gravity, so they can stay as long as they like. This reduces what you have to launch significantly.

It makes keeping people on the station closer to maintaining an Antarctic research base than the ISS. While getting them there is still costly, many more people can stay on the station for months or years at a time. For example, Amundesen-Scott maintains 50 people over the winter.

But people are inefficient. They require food and water and sleep and have to return to base. Instead, use cheap quadcopter drones to do the exploring. They can be controlled by the people on the station, or by the people on Earth, or in a semi-autonomous mode. Drones are light and cheap compared to food and water and humans, so you can send a lot of them. Have the humans there not for exploring, but for drone maintenance and to do the odd thing the drone cannot do.

How long will it take to explore your structure? With sufficient drones exploring every path in parallel it will take as long as it takes to traverse the longest path. If your station is laid out with any sort of logic, this will be the time to go from one end to the other, plus time for doors. Room exploring time does not matter except to find the exits, follow on drones and humans can explore what is found in parallel.

Just how big? At a very modest pace of 1km/hour, 100 years (or 36,500 days or 876,000 hours) means the longest path must be 876,000 km. That's over twice the distance to the Moon. If the hallways are roughly 3m x 3m (space for a person to walk, plus piping and wires and structure and such), and we assume the twistiest path possible, that's an internal volume of 7,884,000,000 m3. or 7.9e9 m3. How big is that? For comparison the pressurized volume of the ISS is about 1000 m3 or 1e3 m3 or 8 million times smaller. Reaching for the handy Orders Of Magnitude By Volume (I love these lists) we find it is about the volume of Lake Thun, a rather large lake in Switzerland. And that's the best case using the most twisty and most nonsensical path.

Making it so big it cannot be explored by drones in 100 years is impractical. You either wind up with a ludicrously large station, or some ridiculous reason why you can't use drones, or an internal layout that makes no sense.


Now, why it will always been an object of interest.

If you want to leave something unexplored, do what archeologists do: be wildly underfunded. As any archeologist will lament, there's acres and acres and acres and acres and acres of unexplored and unstudied ruins and artifacts out there in the world and not enough money to fund the studies. As any archivist will tell you, museums are full of great stacks of unexamined artifacts and not enough money to fund people to sift through them.

So, like with exploring the Moon, maybe there was initially a great race in the 1960s to land on the station. After a few manned missions they didn't find the great wealth of knowledge they expected, and public interest and funding waned. Since there's gravity it can't even be used as micro-gravity research and manufacturing.

Later, with the advent of cheap spaceflight and cheap drones, universities can scrape together the funding to send one drone to the station, similar to how they can launch micro-satellites now. Other exploration is done by the odd commercial venture to explore the station in search of rumors of those lost ancient vaults of knowledge that must be there somewhere.

...but even this is absurd. A large, human-friendly, orbital structure has enormous economic value. It would be an enormous boon for the space mining and manufacturing industry. It would be used as a fuel and supply station and orbital construction center. Asteroids could be brought nearby for mining. Satellites could be serviced. Spaceships could be built and launched from orbit saving tremendous amounts of fuel and mass.

Looking at it from the Cold War perspective, it is the ultimate high ground. Whomever controls the station can threaten to drop a rock on their enemies. No major nation would allow it to remain unoccupied.

From another perspective, this station is the answer to the eternal question "Are we alone in the universe?" It's a big blinking sign saying "NO!" Of course it's going to be explored to find answers: Who built it? How did they interact with us? Where did they go? Can we go there, too?

Once we can reach this station, we would have no reason to leave. It would be our jumping off point to the stars.


Finally, how to make it unexplored.

Where on Earth have we not explored? Underground and deep under the ocean. Why haven't we explored there? Heat and pressure. Since it's a space station, you can throw in radiation.

There's lots of reasons why parts of the station (or the whole station) might be too hot, overpressurized, or irradiated. Maybe the environmental controls are broken. Maybe it wasn't built for humans. Maybe the power source is overheating. Maybe the radiation shielding has broken down.

As we learned at Chernobyl, even robots do not like working in high radiation environments, and even robots have to vent waste heat. This would severely limit how far and how long robots could explore parts of the station.

We can't fix it because we don't understand the technology. To us, their technology looks like a microchip would look to a 19th century scientist: a mostly featureless lump. All their essential systems are presumably encrypted far beyond our capabilities. Some of the critical systems are in the hostile zone. We don't care try to disassemble any of it because it might make things worse.

We don't understand their language and have no Rosetta Stone upon which to start a translation. If it is an alien language, we have no basis upon which to perform an analysis. We'd have to try and map glyph (if they even use them) to their functionality of technology we don't understand.

Slowly, painfully slowly, we'd do careful experiments to pick apart how (hopefully) minor systems work to learn the fundamentals. We'd develop new instruments and new areas of science and physics to analyze their tech.

And then, maybe in 100 years, we'd learn how to fix the station, reduce the hazards, and finally get a look at the core.

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    $\begingroup$ Just because people are stationed on it, doesn't mean they're exploring. We could land, look around, set up a base, and then stop exploring. The people on the station are busy mining and repairing satellites. They don't have time to look at dusty corridors and empty rooms that all look identical. The people who actually explore are the ones underfunded - perhaps deliberately so, as they get in the way of the mining, manufacturing and supply industries. Add in corporate ownership of the structure, people who don't want an accidental discovery to jeopardize their claim to it ... $\endgroup$ – R.M. Apr 15 '16 at 18:38
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    $\begingroup$ @R.M. Why would they stop? They're standing on the answer to "are we alone in the universe" and they'll just ignore it? And why would it be turned over to a corporation? $\endgroup$ – Schwern Apr 15 '16 at 19:14
  • $\begingroup$ I would like to point out that you could easily remove drone exploration by just placing pressurized doors, as any drone strong enough to turn the wheel to open the door, would have to be heavy and complicated. $\endgroup$ – Vakus Drake Apr 16 '16 at 17:00
  • $\begingroup$ @VakusDrake That's a good point. Obstacles make things more complicated, but it doesn't stop drone exploration. Quadcopters explore the space they can. When they encounter an obstacle they cannot surmount, they call for a specialized unit to deal with it. This could be a human, it could be a specialized robot (something regular like a door can be designed for). Doesn't have to be particularly heavy or complicated, just able to firmly plant itself and have sufficient mechanical torque. Meanwhile, the quadcopter explores somewhere else. It will slow things down, but not 100 years down. $\endgroup$ – Schwern Apr 16 '16 at 18:19
  • $\begingroup$ Yeah see I'm not so sure about it being simple, for one he gave no indication of very good computing in this setting so it would probably have to be remote controlled (given likely 1950's level ai) and if you are already controlling it, unless it's dangerous, your time would be better spent exploring stuff yourself. Also keep in mind the radio-control range would be smaller than normal since it has to go through what is almost certainly super reinforced bunker walls maybe every 10 ft on average. $\endgroup$ – Vakus Drake Apr 16 '16 at 18:39
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With 1970's to current space technology, the station would remain largely unexplored for thousands of years even with continuous, dedicated efforts to explore it.

Our moon has a radius of ~1700km. Assuming a "smaller" artifical moon had a radius of 1000km, it had a volume of ~4 million km³. Further assuming an average room/corridor height of 4 meters, accounting for high ceiling halls, outer walls etc., you had to explore an inner surface area of roughly 1 million km², equivalent to the surface area of Egypt. In addition to the average 4m height of rooms and corridors, further assume an average width of 5m of corridors, where a room is defined as a broad corridor. That would require the exploration of 200,000 km of corridors. Since we are talking about rooms and corridors for the most part, you will only ever see as far as the farthest wall, so whatever area you want to survey, you will have to physically get there. No tool more sophisticated than rangefinder binoculars will aid in the exploration in terms of covering raw area.

Human walking speed is 6 km/h; assuming 8 hour work shifts plus breaks, a human could cover ~50 km/day. That would mean 4,000 person-days, or 11 person-years to explore the whole station by foot under ideal circumstances. In practice, this is based on a number of unrealistic assumptions:

  • No backtracking whatsoever: Assuming the station layout even allows an Eulerian path or a division of the station into multiple such paths, the explorers would have to carry all their provisions and equipment from start to finish, which is obviously impossible. Alternatively, someone has had to go ahead and create base camps, which already implies backtracking has occured. This factor alone will increase the time to fully explore to several times the theoretical optimum.

  • No multiple exploration runs: Locations deemed interesting will have to be visited and investigated multiple times with specialized equipment to count as explored.

  • No obstacles: The question already mentioned the station being in various states of disrepair and poor lighting, which slows down the average speed of exploration, increases the need for additional tools such as lights or debris cutting/moving tools and requires detours for truly impassable obstacles.

  • Competition: Since we are talking about a cold war era, political tensions and the direct rivalry of space-faring powers could easily prevent the complete exploration of the station for an indefinite length of time, and might even make the station a battleground.

  • Simple logistics infrastructure: No matter how many person-years you assume the exploration to require, you have to at least double or triple that number in order to support the actual explorers. They have to eat and drink, require equipment and maintenance thereof, medical assistance, command and communications, maybe even military protection from competing powers. Triple it again because everything happens in space.

Overall, assuming the political background does not significantly obstruct the exploration into impossibility, we are looking at several hundred person-years as an absolute minimum for exploring the whole station. Let's make it 1000 person-years.

In practice, space technology and space-faring economics will be the most limiting non-political factor. The Apollo program managed to put a total of 18 people on the moon, for an average of 10 days each, over a time span of ~3 years from the first landing (Apollo 11) to the last (Apollo 17), or 60 person-days per year. At 1000 person-years required, this would require a continuous Apollo program with a duration of ~6000 years.

Modern space technology will not significantly improve the economics of going to the moon or its smaller brother. The rocket equation still holds as true today as it did in the 1970's. You will still use oxygen and hydrogen as your fuel. You will still have to use multi-stage rockets made of light and durable metals. In order to fundamentally reduce the costs or increase the frequency at which you can send stuff to the moon and back, you would need novel ways of doing so, such as a space elevator or a space fountain.

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  • $\begingroup$ Regarding your last paragraph, that is why I specifically mention Orion and NERVA - both are high thrust, low fuel solutions which would dramatically change the parameters of the rocket equation. $\endgroup$ – Moo Apr 15 '16 at 15:19
  • $\begingroup$ @Moo define "dramatically". Even if you could triple the efficiency, we still would be talking about 2000 years by my numbers, still impractical. I still think that gradual improvements to chemical rockets would be insufficient for detailed large-scale exploration of a small moon such as you described. $\endgroup$ – Hackworth Apr 15 '16 at 16:15
  • $\begingroup$ What about robots? $\endgroup$ – Schwern Apr 15 '16 at 18:35
  • $\begingroup$ @Schwern Inferior to humans even with present day tech, let alone 1970's. Today's autonomous robots can barely open doors on their own. In the long run, they will certainly surpass humans, but not for the forseeable future. I once read somewhere that a human geologist on Mars could get more work done before breakfast than the mars rovers during the whole day. $\endgroup$ – Hackworth Apr 15 '16 at 18:53
  • $\begingroup$ @Hackworth Why do they have to be autonomous? $\endgroup$ – Schwern Apr 15 '16 at 19:13
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I think you'd need to add a condition - something like once under the surface, remote drones or fast vehicles can't be used (insert reason of your choice) because otherwise you could very rapidly explore using small drones.

So with that out the way you are down to people exploring - and the obvious challenge is getting people there. It is incredibly expensive (in terms of cash, resources and people) to get a tiny number of people to the Moon and back. You aren't going to get an order of magnitude improvement on that by 2050.

So assume only a small, select group of visitors each mission, and missions, even if you throw resource at them, will not be able to happen continuously - you'll run out of resources, or you'll have an incident that while pause space exploration while faults are understood and fixed (see evidence from the Space Shuttle...) and even something a tenth the size of the moon would not be able to be explored fully. Remember volume goes up as the cube of the radius - there's a lot of volume in a sphere.

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  • $\begingroup$ Comparing it to the difficulty of the moon landing discounts the enormous benefits of having a human-friendly environment at the other end. You only have to haul up enough bulky environmental systems (air, heaters, radiation shielding) for the trip there and back. The people can stay on the station as long as their food and water holds out. $\endgroup$ – Schwern Apr 15 '16 at 18:33
  • $\begingroup$ One such reason that would seem likely, would be interference with radio communications. A high-tech station that's active might very likely monitor all EM radiation, and detect, try to communicate and/or interfere with information signals such as drones use. There might even be an "unauthorized electronic/robotic device" system that shuts down drones even if they are fully computerized & self-contained. $\endgroup$ – Dronz Apr 15 '16 at 18:36
  • $\begingroup$ @Dronz (Ironic name to be arguing against drones). Relays and semi-autonomous modes can take care of interference. Otherwise I'm not keen on using convenient magic plot devices to shut down selected electronics. $\endgroup$ – Schwern Apr 15 '16 at 19:34
  • $\begingroup$ @Schwern As long as the station allows it, I agree. But if the station is technological and much more advanced that we are, I think it's plausible/likely (but yes, up to the will of the author) that it might be set up to detect something like an unrecognized drone or robot, and shut it down. A technological society needs to be able to handle primitive robots, though you're right it's perhaps even more likely an abandoned station might not do that. $\endgroup$ – Dronz Apr 15 '16 at 20:32
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At first, the great powers rushed to explore the structure. But then the incidents happened. Exploration was immediately halted and only restarted much later, but newfound caution - or fear - meant it never recovered its initial gold-rush pace.

What are the incidents? Don't know, but that's a plot in itself, really. Whatever happened ("Icarus incident" has a nice sci-fi ring to it), it has to be wild enough to scare everyone good. It sounds like whoever built the thing aren't around anymore, so perhaps the explorers learned a bit too much about their fate - and learned it a bit too directly - and it wasn't pretty.

If, at the same time, the structure has already rewarded the initial explorations with more data than anyone knows what to do with, things could settle down while that's being chewed on and interest wanes.

Of course, some will be clamoring for a return to full-on exploration, but people are saying the same for our Moon - and space in general. But generally, people just aren't interested enough to follow through. In the story, that thing has apparently been up there for eons, and doesn't seem to be going anywhere. So let sleeping dogs lie, curiosity killed the cat and all that. Why spend my tax dollars just to have more astronauts and eggheads uncover another [redacted] and end up [redacted] like last time? Man, I don't even like to think about that whole thing. So sad/creepy/scary/horrible/gross.


Or maybe there's simply a very explicit "keep out!" sign and - for once - humankind takes note. At least somewhat. A problem facing us today, for instance, is how to safely store radioactive waste that'll be dangerous for thousands of years. How do you design a structure that's so secure, or simply so uninviting that a future civilisation (that may not share any of our culture or understand our languages) will keep clear? This is a problem that's been studied, but of course no one knows if any of the proposals put forward will actually deter future-man.

Imagine we found an ancient structure here on Earth. We don't know what it is. We just know it's at least 10,000 years old and appears very deliberately designed by some hitherto unknown civilisation. Would we immediately dig/drill/dynamite our way in? Undoubtedly. Would we stop if we discover that those warning sign-looking things are in fact warning signs, and the structure is actually meant to keep us from unleashing tons and tons of self-replicating, radioactive ebola nanites? No, we'd probably keep digging, because as a species we're not terribly smart. But there'd certainly be a lot of debate, and exploration would be conducted very, very carefully, and halted again and again. And a lot of resources would likely be devoted to keeping people from exploring because of the dangers it poses. And sooner or later, the exploration will be so slow that it's just not interesting, and is considered a waste of money.


And, as others have already pointed out, exploration can move in fits and starts. The vikings appear to have sailed to (and made non-permanent settlements in) North America, but everyone remembers Columbus, hundreds of years later. And it took another hundred years and a few decades before the pilgrims landed at Plymouth Rock. A then a few hundred more before Lewis and Clark. Et cetera.

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Some things: depending on what you want the tone of whatever it is you’re making this for to be, a pretty easy way to validate decades of exploration is to make sure it keeps changing. If you want to keep it within the realm of plausibility, then it would probably make sense for this object to be self-maintaining, given that apparently it’s been up there for most of human history. This would probably include repairing, maintaining, and building new things, as well as destroying old ones. Staying in orbit is one of those things that’s really easy for a big ball of rock, but really hard for something that humans which need to stay on the surface of another big ball of rock can survive in. Because Space is Hard.

If you want to go with the slightly more supernatural method, then why not throw in some non-Euclidian geometry? No one really seems to know what exactly this thing is and/or where it comes from, and it’s mostly unexplored, so it’s not like all the humans back down earth have a good reason why it shouldn’t be bigger on the inside. It can be advanced technology, space distortion, general weirdness, whatever. This works, because historically, most Big Dumb Objects are supposed to be pretty weird.

Also, exploration time really depends on how involved someone is in the process. They could carefully measure, test, and photograph every single door or passage they find, which is actually pretty likely, at least at first. But this would also probably take forever. The other end is you could just draw some rough lines on a map, and call it a day. Or, just wander off and see what you find! Historically, this is more or less how humans found new places for pretty much ever. They put a bunch of guys in a boat, and told the boat to go somewhere and try and keep track of what direction they were going in. eventually, they would hit some beach or another, get out, figure out roughly where it was based on where they were pointed when they started the voyage, plant a flag, get back in the boat, and go home. This is good for filling in rough outlines, and testing things like “Maybe you can’t fall off the edge of the world?” or “I’m pretty sure that India is this way?”

Then, after some people have lived there for a while, things start to get narrowed down. Rivers and mountains are found and named, then they work downward, to streams and hills, to creeks and valleys, until every other little aspect of the landscape is all mapped out. This takes a really long time, which is why we’re still doing it.

So, I guess depending on how “discovered” something is, speed depends on how familiar it actually is. More habituated areas would take longer to map out, while new excursions are pretty cut-and-dry. Of course, this is all kind of irrelevant if the whole thing keeps changing, but there’s no reason why some parts could change slower than others, or not at all. It could also just get harder the further you go, less hallways and more locked doors. Exploration is really just dependent on how thorough you’re willing to be, and how much stuff is in your way.

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  • $\begingroup$ I'm getting a serious "Cube" vibe off this answer. $\endgroup$ – Schwern Apr 15 '16 at 18:31
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There are some very good answers, but I think they are to pessimistic in view of your edits.

  • The station has breathable air. I assume it has water as well, and that humans figure out how to get it. The environmental systems would have to pull humidity out of the air. Public restrooms? Parks and fountains?
  • Can humans figure out how to draw power from the station systems? If there are no easily accessible sockets, perhaps they will learn to vandalize light fixtures.

From these, I expect that hydroponic gardens are possible. Bring plants, use the water, use human excrement to supplement the initial load of fertilizers. This will greatly reduce the supply problems if you eyeball the list of supply missions to the ISS.

  • The station has gravity. I take that as a close-to-earthlike gravity which allows people to stay healthy for long periods. If transport costs remain high, people could serve for decades at a time. Think of Brits going to India in the 19th century.
  • The Apollo program went as high as four launches in one year. Assume that level is sustained all through the 1970s, and nobody goes back. 660 man-years.
  • The Space Shuttle program suffered setbacks. Assume that a level of ten launches per year is sustained through the 1980s and 1990s, each with ten passengers in addition to the crew. Again the passengers don't go back anytime soon, but we'll allow the Apollo-era crews their first shore leave. 5,500 person-years in the 80s, 15,500 person-years in the 90s.

This gives a total of 21,660 person-years to explore the station. Say each person-year gives 300 workdays of 12 hours each. That should be low enough to prevent mutiny, high enough to satisfy the taxpayers back home. That accounts for 2,120 people going into space, 120 on Apollo capsules and 2,000 on shuttle flights. As you can see, the contribution of the Apollo flights isn't all that great, so it won't change the total much if they spend most of their time developing techniques to live on the station.

50% of the work goes into "life support" for the expediton. Running those hydroponics, darning old socks instead of using new ones, transport of food and supplies where you need them. Another 25% goes into "starside R&D" like figuring out how that subway works or why there is gravity.

That leaves about 20 million hours of "exploration and mapping" duty. Now you need assumptions about the average room and corridor size. Imagine a four-person team moves forward at 1 km per hour and covers a corridor with adjoining rooms, 20m wide and 4m high. Each person-hour explores 20,000 cubic meters of the station. That means 400 billion cubic meters are explored.

(For that rate of progress, I'm assuming they will open most doors and just glance inside. "Yep, another family apartment. Just like the last one. Write it up and go on.")

A billion cubic meters is a cubic kilometre. So they could explore a cube roughly 7.5 kilometres on a side before the turn of the century, and without any new launch vehicles.

The design of the station would affect this greatly. Just doubling the average ceiling would double the mapped volume. Of course there would be expeditions to the center of the station, and so on, the mapped area won't be a cube.

A 120-km sphere like the first Death Star would do nicely to leave plenty of unexplored space.

BTW, I haven't talked about robots yet. They might be able to help with the initial survey of major corridors, disused subway tunnels, etc.

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The Great Pyramid in Giza is roughly 91 million cubic feet in size. It is estimated to be around 4,600 years old and we still haven't fully mapped it out.

So, taking into account the #3 and #4 requirements I'm going to estimate that the structure would have to be no bigger than the Great Pyramid. Which, obviously, is far far smaller than the moon currently is.

The issue isn't in how fast a team of robots could go through and build a virtual map of the station. The issue would boil down to conflicts between political, religious and scientific entities.

Some simply wouldn't want it explored as that might be seen as "desecrating" a holy site. So, those people would have to be appeased by going slow and being extremely careful in how results are published.

The scientific community would have legions of people trying to build their careers on coming up with what everything from the smallest scratch means to what the rooms where even used for (hint: archaeologists default position is to always claim religious activities). Also, these people would want to be as sure as possible that each room they enter is left in the same state as absolutely possible so you can forget scanning a room, then moving onto the next one as quickly as possible. And god help them if they find writing of any sort anywhere.

Politicians would want to ensure that their particular countries/states/etc would benefit from the exploration so it'd take a while to put together teams, assign them, replace them, etc. There would be work stoppages as discoveries are made and the benefits/conflicts from those discoveries are doled out.

Finally, we have the ever evolving nature of the global political situation. As countries go to war or find themselves financially unsound, work would necessarily stop until those issues were resolved. Also you can pretty much guarantee that if the thing is of a sufficiently large size that one or more countries would be planting a flag on it and claiming total dominance....

Yeah, 100 years to explore an ancient structure isn't a whole lot of time.

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