Funny you should mention that.

IMHO a fairly efficient and cost effective way of constructing a shelter against various situations by paying a visit to your local pre-fabricated concrete distributer with an eye toward their larger septic tanks, cisterns and manholes.

A large septic tank can be 10' long, 8' wide and 6'6" high inside. Plant it anywhere from 3' under and half-way exposed with a standard manhole bumped up against one end and you have a workable shelter. If it is half above ground, maybe because of a high water table, you simply berm the exterior.

I'm leaving out a lot of details, like drainage and how to join the main pieces so they stay together and don't leak, but anyone good with their hands and familiar with these materials can do a workmanlike job.

It can be practical DIY job for advanced practitioners. You can rent a backhoe for a weekend at about $200 and dig the hole. If you set it up right gravel cam be poured right into the hole and compacted. Most pre-fabricated concrete manufacturers deliver with a truck that can drop the structures in their final position if they can get close enough to the hole. The key here is to talk it out in detail with the people your buying from and find out what they can, and cannot, do.