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.

Lava oceans
While the planets may 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 non-core ocean of it), as well as at the bottom of some Gas Giants.