For a no-kidding tornado shelter I would recommend you check out www.monolithic.com. Yes, the concrete domes are butt-ugly. However, FEMA has rated them as "near absolute" protection. That is the highest rating they have. I'm serious, check it out. I am actually considering building a house using this construction technique.
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"Its not a matter of being ready as it is being prepared" -- B. E. J. Taylor