The title almost say it all. But to clarify a bit - I don't mean building a Base inside a large cave (would also be amazing), I mean having modules that are designed to be "buried" under ground. Dugout rooms and corridors etc. https://robertsspaceindustries.com/spectrum/community/SC/forum/61894/thread/modular-base-design-above-and-below-ground-ikea-st/171218