Subterranean Structures?

What would be some good ideas for subterranean housing structures? And why?
Not just another basement layer, but an actual housing feature, of which physical location would be under the house plot and garden.

Examples of subterranean structures which might happen under a house and garden are small nuclear bunkers. (Which would never happen in our game but as I said ... just an example xD)

Wine cellars, storage facilities, vaults, ancestral tombs, secret laboratory/workshops, bunker (in case of calamity)....