[Dirk Gently]

There's a setting in the XXI's zon file that makes it crush extremely fast when diving below max depth. It takes 50 points of damage per second where most subs have 2 points of damage per second.

The hull itself has more hit points and more armor than any other sub in the game.

EDIT: I'm doing this from memory (a decade) and I noticed someone sporting some different numbers up here so I might be off. But I remember it being so much higher that it baffled me.