[@Draconequis] I suppose that's a fair point to make. I guess chances are that it's more cold than hot throughout the Underlake, under those circumstances... then again, the Underground did have a variety of biomes, including a lava area at an inexplicably higher altitude than a frozen area (we presume, since Frisk was most likely moving upward through Mount Ebott overall); I suppose it's not inconceivable that Lake Mirth's cave system has lava somewhere in it for some reason or another. And with that said...

[hider=The Hot Springs]
[img]http://pre00.deviantart.net/0669/th/pre/i/2013/055/b/4/lava_cave_by_daniseik-d5w1r4p.jpg[/img]
Really, REALLY hot springs. It's a joke, because only the sturdiest and most well-adapted of monsters could survive a dip in magma, and the temperature is so vastly different from most other locations that few dare to visit. Even so, there [i]are[/i] monsters living in the area, and the fast travel submarine is somehow capable of moving its passengers through the dense superheated rock. Bring plenty of water.
[/hider][hider=Entrances and exits to Hot Springs]
[img]https://oneworldoneheartbeating.files.wordpress.com/2011/12/inside-the-lava-cave.jpg[/img]
Somewhat less hot than the Springs themselves, but still dehydrating to move through. Tiny streams of magma flow beneath the surface of the rock, making for a very pretty light show... just don't touch them with your bare skin. Or anything flammable, really.
[/hider]