Lava

Lava is a very dangerous liquid. You will take a lot of damage (30+ per second) if you step in it.

It exists in small puddles/lakes at the bottom of most planets, including the starting Garden planet. It can be centrifuged for early Tungsten Ore.

Thermal Shell EPP and Burn Resistance Spray provide immunity to lava, allowing you to swim in it.

Lava oceans
Many planets (including the starting Garden planet) have a core ocean of infinite Lava. It wasn't minable in vanilla Starbound, but can be mined in FU.

There is also obtainable infinite Lava on Magma and Dark Magma planets (which have a surface-layer ocean of it), as well as at the bottom of some Gas Giants.