Lots of good ideas here, looks like they are making pre-fab "storm shelters" with NBC filtration options available. Looks like a better solution than others I've seen, however they are a bit spendy, as the accessories they offer can be had elsewhere for much less. HOWEVER a 1" thick steel hatch + lift jack is hard to do unless you have a welder and plenty of time.

http://www.f-5stormshelters.com/options/index.html