Culvert as cellar/bunker? Why not?

Has anybody here built one, or seriously considered it then ruled it out for whatever reason?

I've always been interested in the idea of building a hidden shelter by having a large (10' or 12') culvert shallowly installed across a hillside. My area is steep and hilly enough that 50' or 100' of culvert could penetrate all the way from one side of a ridge to the other.

It seems appealing for the engineering simplicity/reliability and convenient access. Culverts seem like the state of the…