An Offer You Can't Refuse
"Were it a normal tunnel, built by a normal culture, absolutely," said Book. "Given what I know of their build methods and the explosives you compromised on, it would take an extraordinary amount of skill and more than your share of luck to effectively collapse the entrance. You certainly don't need to worry about upsetting the sea cave. Pulling it off, and fooling any demolitions experts who may be watching into thinking you took out the whole facility? That'd take a miracle. And they'll be watching. Both of them. Unless they've got something more immediate holding their attention. Might wanna make that a certainty if you want to avoid fighting either of them. Though I have little doubt that, together, it's a fight you'd win."
"Don't count out the Tremere, either," he added. "If I wasn't so confident in Robyn's ability to break the seal, I'd have approached them long ago. But they're resourceful, they're desperate, and they know there is power to be had in those catacombs. People like that beat the odds all the time. You can safely assume that they'll be guarded at a minimum. If you're really unlucky, van Fokke himself might be down there, and you'll seal yourselves in with him."