>> they'd get flooded from the rain! duh.
That would be a problem, all right.
Consider building some sort of above-ground "safe room".
Food for thought: here's a picture of the credit union property in Moore, Oklahoma after the tornado in 2013. A number of people survived by huddling in the vault, which was all that remained of the building:
More food for thought: a book about safe rooms from FEMA, in hard copy or downloadable files: