Magma Fields

The Magma Fields are a randomly-generated location in the Crystal Hollows where Yogs can spawn. When in the Magma Fields, the player's Heat gradually rises until they begin to burn and take damage. This can be somewhat mitigated via which will subtract 50 from your heat, or standing in water, which passively lowers your heat. Heat resistance can also be affected by Heat Armour, Flamebreaker Armor and Armor of Yog (the latter of which cancels heat completely), or via the Bal pet (also completely canceling the effects of heat).

Additionally, lava in the Magma Fields can bypass the Lava and Fire Talisman effects, but not the Campfire Badge effects.

The Magma Fields provide a bottom entrance to the Crystal Nucleus and can house Khazad-dûm.

In addition, it is possible to fish in the lava within the Magma Fields, with a separate loot table to normal fishing.

Trivia

 * Early on in the release of the Crystal Hollows (0.12), lava Fishing in the Magma Fields had vanilla Minecraft loot tables, leading to lots of glitched items.