Man, don't you people have any imagination?
Hi, everyone, by the way. This is my first post here and it's probably not a very good start, but I really had to reply to this thread.
When I was about 15 there was this little lake near my high-school. We used to hang out there a lot. I once freedived to about 20 feet in the deepest part and found the bottom. The viz was 0.0 inches. We (me and some friends) hatched the idea of doing the same thing--putting some kind of habitat on the bottom. Why? Well, hell, just because it seemed cool.
So with no viz, underwater construction was right out. We needed something pre-made. We spent a lot of time looking around for the right kind of thing. Finally we decided the thing to do would be to swipe a small dumpster, clean it out and use it upside-down. Weld plates over any holes and make a bottom for it with a hole cut out to get in and out of. Next, we weld legs on it and put on big I-beams for legs.
How to get it in place. Well, we didn't have many resourses so we figured the thing to do was to drive it out to the lake in winter, slide it out on the I-beam legs (like skis) on the ice, cut or blow up the ice and just sink it there. Then the next summer, we go out, right it and fill it with air from a scuba tank.
Remember, I was 15. It all seemed reasonable.
Anyway, we were working on the issue of air supply. I didn't know how to use scuba yet, but I did have an old copy of the navy tables and it said that at that depth (20') there was no NDL, so we figured we didn't have to worry about that. One possibility was to use scuba tanks and just blow one every so often until you ran out and then just leave. Another possibility was some kind of scrubber, but that was a bit out of our technical range. We weren't sure about using a compressor. We knew that it would have to overcome the pressure difference from the surface to depth, but we really didn't know how much that was. And whatever it was, it had to be quiet or else some damnable grownup was sure to find out and pull the plug, as it were.
But it didn't matter. Because one day I got to thinking. Something really fundamental occured to me.
Things with air in them tend to float.
Hmmmm. I wondered whether that would be an issue. I mean, the dumpster would be pretty heavy, right? So it shouldn't be a problem. But then again, navy ships are made of metal...
So just off the top of my head I figured some things out. Let's see... a small dumpster, just at a guess, something tiny, but just big enough for two people to squat in with some gear... say, 5 ft. by 5 ft. by 4 ft. That's 5 X 5 X 4 = 100 cubic feet. The air weighs so little that for simplicity we can call it zero. Eight gallons in a cubic foot, I think... eight pints in a gallon... a pint's a pound the world around... so 64 pounds per cubic foot. 64 times 100... hey! That's more than three tons of buoyant force!
I wish I could see the look there must have been on my face when I figured that one out.
Well, so how to deal with that. Idea 1: use cables and expansion bolts in the rock. Unfortunately there was no viz to work in and no rock either--just muck. Idea 2: instead of I-beams for legs, use some kind of big trough. Sink the thing the same way, but then before filling the habitat with air, fill the trough/legs with balast. Ummm... three and a half tons of balast. Tungsten would be ideal, but would cost (literally) a fortune. Lead? Better, but still too expensive for the amount needed. Well, obviously the basement bomber solution (I can tell you're the basement bomber type) is just plain rocks.
Horray. We've got that solved.
But wait. Will the roof of the hab withstand 3 tons of upward force on it? Will the rest of the structure?
This was obviously getting complicated and expensive. So it wound up becoming not-acted-upon zany scheme number 8463.
I'd still love to do something like that, but I think it's a pretty serious venture.
Someone say "welcone to scubaboard."
It's been operational for 15 years at least up till now. Not exactly sure when it opened but I first saw it at least 15 years ago. I think since the early 70s. 
