^it's a mountain that's been afflicted with supernatural extra cold for like the past two thousand years and no volcanism to be found in the lore that I can remember. Kind of a miracle there's liquid water there at all, really. The ancient magical machines scattered 'round the place that generate small pockets of warmth might be related.
Doesn't matter to Eula whose affinity for the Cryo element is strong enough that she just ignores cold outright (in fluff, she doesn't get any actual resistances in gameplay :sadge:) and who hangs out there for fun on her free time, and Amber (who occasionally accompanies her on those outings) is conversely Pyro affiliated and can presumably thus readily enough find ways to keep herself warm - she's certainly one of the most efficient portable lighters in the roster... :P