Lava

Lava is a fiery liquid 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 around, losing 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 kill Mario instantly.

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

In the Mario Kart series, lava appears in many race course. Falling into lava delays the racer, as they must wait for Lakitu to pull them out.

Donkey Kong Series
Lava is, like on Earth a naturally occuring and dangerous substance in the Donkey Kong Universe.

The first appearance of Lava in a Donkey Kong game is Donkey Kong Country 2: Diddy's Kong Quest. In this game lava was an obstacle found primarily in the area Crocodile Cauldron, a large volcanically active area.

In Donkey Kong Country 2's semi-sequel Donkey Kong Land 2 lava is slightly less common due to Crocodile Cauldron being fused together with the large and swampy Krem Quay to create Krem Cauldron

In Donkey Kong Country 3: Dixie Kong's Double Trouble lava makes a small appearance in the hidden world, Krematoa. Here a small pond of lava can be seen and quizically swimmed through after Krematoa's ancient volcano is re-activated.

This same pond of lava can be seen in Donkey Kong Land 3, the pseudo GameBoy sequel of Donkey Kong Country 3.

Lava also appears Donkey Kong 64, once again as an uncommon, yet dangerous obstacle.