Lava

Lava is a liguid thing found mostly in Bowser's Castle. It can be found in other parts of the Mushroom World as well.

When Mario touches lava he will either get burned by it and run aorund loosing some Lives (as seen in Super Mario 64), or die instantly (as seen in Super Mario Sunshine).

In Super Mario Sunshine, Bowser Jr., A. K. A Shadow Mario, could paint Goop with his Magic Paintbrush. Some of the goop was lava goop, however it did not make Mario die instantly.

King Magma Blooper from the upcoming Super Mario Galaxy will be a lava Blooper.