F3 can tell the current temp, it is in C.....how it reads 100 C, (40 C player 60 C). First temp reads biome current temp, ( gear temp, then current player temp). The underworld temp is 100 C and very easy to get over heated wearing nothing. Gear/Armor does not help out. Hell Stone and Lava is all there. From the crust to the lava is a depth of 51. At 800 when everything opens up. Pillars do not touch the lava. Easy to fall to death.
Deep underground I found a massive chamber with many tunnels...including throwing a flare and watch disappear in the darkness while lit. Even a flash light could not see the bottom. I found and killed over 10 spiders which were annoying in that one place.