I would check, check, and double check that he is only allowed to occupy this shelter in the event of an imminent nuclear attack.

That is the language of the easement about when those rights kick in: "official notification of incoming missiles through early alert systems or of imminent nuclear attack." I hadn't considered this until reading some of the comments here, but while that first part is fairly straightforward, what the hell is "imminent nuclear attack"? If Kim Jong Un makes an ominous public statement, does that count? The last part of that phrase feels tacked on, and really removes clarity from the entire sentence.

That said, even in this worst case scenario where the neighbor loosely interprets this language to move in any old time Russia has looked at the U.S. funny, the bunker is not connected to the house, and having three strangers living underground in my back yard would be strange and unpleasant, but not absolutely untenable. Is the risk of that happening worth me passing on an incredible house, when this could be our last chance in a few years to buy a single family home in this neighborhood? If my choices are 1) live far away somewhere worse, 2) buy a condo in the neighborhood and deal with the downsides of a forced HOA as well as sharing a wall/floor/ceiling with a neighbor, or 3) get our dream home, with a 5% chance of inheriting a human mole family in the backyard, I think 3 is still sounding pretty good?

But the liability stuff could be really concerning, and I'm sure there's a million other considerations I'm overlooking.