Okay, so the hidden earth-sheltered habitat we saw last week works pretty well when it comes to not being found. But what if you want to go one step further? After all, someone sniffing around your land will eventually find it if he persists long enough and stumbles upon the entrance. And also, if anyone visits your location while you’re building the thing, it’ll be obvious what you’re doing.

The only real solution to these possibilities is to go completely underground, both during the construction and the operation of your building. This will obviously take longer, be more complicated and involve some danger, unlike all the previous concepts we’ve covered. But since secrecy is a crucial aspect of what we’re doing, in some scenarios it’ll be worthwhile to explore the possibility of completely underground construction.

Tunnel digging

At the core, building underground is about tunneling. And while digging is simple, not dying in a cave-in is less simple, and something you’ll want to avoid.

A underground hideout will be constructed out of a series of tunnels linking larger rooms together. For illustration, observe the tunnels built by the Vietcong during the Vietnam war; the tunnels were dug by hand, had rooms to live in, storage areas, medical centers and so on. The Vinh Moc tunnels in particular are a great example, since they were used to house civilians as well as military staff, and were buried 30 meters underground to protect them against American bombings. If building such a thing is possible during war time with no resources, then it should also be possible for us to do it in our advantageous conditions.

The Vinh Moc tunnels, and Vietcong tunnels in general, were excavated by hand and did not use wooden supports. This is because the soil was mostly sandstone, which is solid enough to avoid cave-ins if the tunnel geometry is intelligent, while also soft enough to dig by hand quickly. If you find a location with sandstone soil, you can directly copy the method of the vietcong tunnelers, without too much risk. But in many cases, additional security measures will be necessary.

Support

The most basic form of support is to shape the tunnel into a vault as you dig it. A vault, by virtue of it’s geometry, has a very high ability to withstand a load. Thus a vaulted cavity is much less likely to cave in than a cavity with straight edges. But this will only work if you have a soil with very high clay content (or you’re digging through stone).

In order to dig a tunnel into a proper vault shape, you’ll want to create a frame (a bent metal rod will work) with the right curve, and place it against the wall you’re going to dig into. Use a tool to mark the curve into the soil, and carefully dig according to that mark. Every few feet of digging, reapply the frame to make sure sure you’re staying on track.

As an added security, make sure there’s plenty of soil above the ceiling. Some sources say that there should be twice the thickness of earth above the tunnel as the height of the tunnel itself. Digging a shallow tunnel is inviting a cave-in.

But what if the soil you’re digging into isn’t so strong? You’ll have to add support beams as you dig, at least every 6 feet or so (2 meters). Of course, if you start to come across less stable soil, you might have to increase the frequency of support braces to avoid a cave-in. Tunneling braces are typically made with 6×6 inch lumber, but you can probably do with small trees cut around the construction area if you’re low on resources.

An interesting alternative, which I’ve thought about, but which is untested as far as I know, would be to dig a vaulted tunnel and erect a ferrocement shell over the ceiling and walls after every few feet of progress. Ferrocement, as mentioned before, is almost as strong as steel, is totally waterproof, and when built in the shape of a vault, should endure enormous amounts of pressure. It’s a concept that has definite potential, enabling strong tunnels to be dug in even very poor conditions (sandy soil, shallow depth, etc). This will have to be tested however.

Finishing
Living in raw mineshafts, while romantic, would probably wear thin after a while. So it’s a good idea to finish the floors, walls and ceilings with some sort of plaster to make the tunnels more habitable. A simple compound to plaster with would be a mix of cement and dirt (1:4 ratio), which will make your walls smooth with a nice beige color. If you used the ferrocement shell method, then you could just paint the cement surface, which should already be smooth anyway.

The vietcong tunnels didn’t feature doors, apparently. Indeed, it would be very inefficient to dig passageways large enough to accommodate a standard sized door. Not to mention, standard doors are square, which will not match our vaulted corridors and rooms. A simple substitute would be to just use drapes as doors, hanging from the top of the doorframe. Otherwise, you’ll have to make your own doors with appropriate shapes.

Since our ideal is stealth, windows are obviously out of the questions. Light wells are possible, but are difficult to build. So for the most part, using energy efficient led lights will be your best bet, combined with well hidden solar panels on the surface.

Systems
As you dig deeper into the ground, surface temperature stops influencing the tunnel’s climate and you start to get nearer to a constant 13 degrees celsius all year round. This is a cold but livable temperature, if you’re well dressed, making heating not entirely necessary. But building rocket mass heaters will still make the place more comfortable. It would be a good idea to heat at least one larger room, for eating and relaxation. The heater can double as a cooking stove. You’ll probably want to have a propane cooker as well though.

Rudimentary toilets can be implemented by simply digging a deep hole and putting a seat on top. Composting toilets are also a possibility, but an extra hassle (you’ll have to have storage space for 6 or more waste barrels, because it can take months for the human waste to become compost). We may cover the subject of composting toilets in greater detail in a future article. Stuff like septic tanks and drain fields are out of the question underground since you won’t have an excavator to make the gigantic holes necessary to install such a system.

Ventilation is essential for any building, but doubly so for underground construction. The potential for condensation and mildew is very high in a shut off bunker. Good airflow will prevent that. You’ll want to implement a convection based ventilation system. Basically, you get a pipe running below the tunnel for a bit, where it’s colder. The air in the pipe will be cooled. Then the pipe goes to the surface, where it’s hotter. The temperature difference will create a suction effect, and thus an airflow. In the winter, where it’s actually colder outside than down below, you can help the convection along by painting the chimney black and placing it in view of the sun, making it heat up even if it’s freezing outside.

Conclusion

Digging a secret underground network of tunnels is both dangerous and difficult. It’s not something that should be done lightly. But it’s a proven strategy in times of struggle. “Tunnel clearing” was probably the most feared and dangerous type of battle for the American forces fighting during the Vietnam war. That’s something to think about.

As with all the articles in this series, this isn’t meant to be a complete guide, but just an overview and a taste of the subject. Tunnel digging, because of the dangers involved, should be the subject of particular research and preparation.

And as such, tunnel digging should be the last resort. Building above-ground structures, or even earth sheltered ones, will be faster, easier and less dangerous, and it will result in more convenient habitats. But if the heat from the system is too intense, even the most well hidden and well camouflaged structure can be found, especially if helicopters and spy satellites are involved. A hidden tunnel entrance could only be found by large teams going over vast expanses of land with a fine tooth comb. Something else to think about.