@Zarus
GURPS Underground Adventures (which tries to approach underground facts close to how they are in reality, through a playable lens) recommends starting cave temperature as nearing the yearly average of the region, then says that temperature rises 1ºF per 82.5’ of depth, or 64ºF per mile, and notes that deep mines [deep dungeons] are likely to need massive artificial cooling. And some places may be
hotter.