Storm Shelter Made from a School Bus

I saw on the news today that a couple bought a school bus for $800 and buried it underground (another $800 to dig the hole), and it serves as a storm shelter for their mobile home park.

Living in tornado alley, we've been contemplating some sort of storm shelter. I mentioned this concept to my brother and he's intrigued. He said our schools auction off old buses frequently so getting one wouldn't be difficult. He owns a backhoe, so digging the hole would be free, except for the fuel.

Our question is, how do we waterproof the bus? I can't find anything on the internet about waterproofing an underground bus. My brother is afraid water will filter in and mold will set up.

Have any of you done this before? If so, how did you keep it dry? The last few years, our springs and falls have been so wet, to the point of disastrous flooding. How would we keep water from seeping in with each rain? We can caulk windows and doors of course, but what about other crevices that we can't get to, like under the hood and dash? Maybe we just wrap the entire bus in tarps?

Any ideas?

NancyAnn