It's a salt dome underground. You could drop bombs on it for months and not crack the dome.

They drilled into a salt dome in Mississippi, placed and detonated a nuclear bomb into it to see "what if". They didn't destroy the dome. It created a nice little pocket of highly radioactive salt that self-capped with melt material. The cavity got bigger, but the salt dome is still intact.

That kind of durability is what makes these kinds of structures perfect for geological storage.