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
While many planets (including the starting Garden planet) have a core ocean of infinite Lava, it's not possible to obtain it (nothing will work on it: Matter Manipulator, pumps, etc.).

However, there is 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.