Rock is not very compressible. The cavities created by underground nuclear tests aren't that large: https://en.wikipedia.org/wiki/Underground_nuclear_weapons_te

Rock is not very compressible. The cavities created by underground nuclear tests aren't that large: https://en.wikipedia.org/wiki/Underground_nuclear_weapons_testing#Effects https://en.wikipedia.org/wiki/Underground_nuclear_weapons_te... . They did get surface subsidence, but maxing out at a scale of "over half a mile", and not terribly deep. To create "huge caverns", that volume still has to go somewhere, and when you're talking about vaporized rock that mostly means atmosphere or nothing, more often the latter. But at least they didn't always get fallout, that's nice.