The Deep
Below everything is a layer too hot to be in. The rock turns dark red, caverns open out, lava pools, and it starts killing you.
You know you've reached the deep when the rock changes to Hot Stone, shot through with red, and the caves get big. There's a margin below that line, so you can walk into the dark red stone, see it, and turn round.
It's the one hazard in the game gated on equipment. You can't dig or build your way out of it: no amount of hull or stone helps. A Heat Suit stops it outright, not just partly.
Going down
- A Heat Suit holds about 4 minutes of protection: one trip down, grab what's in reach, come back.
- Pack Cells on your back top the suit up. Two of them and a Heat Suit is most of an afternoon in the lava.
- The Insulated Suit holds about 20 minutes and is the late-game answer.
- A Geothermal Tap down here is the steadiest power in the game.