There are so many containers here because they are cheap to produce overseas and with the trade deficit there are a lot more loaded ones coming than loaded ones going back. I guess it's cheaper to leave the older ones here and buy a new one than haul them back. Also, they get a hole or something in them, you're paying "Bona Fide Union Laborers" to weld a patch over it, that patch probably costs 3 times what the container runs new in Shanghai.
It's just a big metal box, and the corners have sockets that attach them to each other or the trailer, stacking them is easy with the right equipment. A bunch of steel welded to the exterior would reinforce them to be buried, look at the "Doomsday Bunkers" they build on the TV, you could have the same or better for less than 1/4 of their prices.... Some rebar, mesh, and concrete and they are pretty much proof against most people without explosives. Spot weld some studs to the walls and use those to fasten 2X4s to the walls, throw on some wallboard and from the inside you'd never know what it was made out of.