I'm not entirely sure but maybe the Vault? They deal with that in Mike Careys run.Little question for the historians.
I vaguely recall the start of an X-Men story where they somehow detect an underground chamber dating back from god knows when, without entrance to it. And the kicker is there seem to be someone in.
Now are my memories wrong ? Was it good ? Who the fuck was in there ?
Also, X-men related but "Inferno" is good stuff. It's always fun watching Xavier get completely owned.