Lava
Lava is a fiery liquid. It can be found in many parts of Mushroom World as well.
When Mario touches lava, it can scorch him terribly, causing him to run around uncontrollably in pain. In some instances, it may result in him losing some lives (as seen in Super Mario 64), or instant death (as seen in Super Mario Sunshine).
In Super Mario Sunshine, the Goop painted by Bowser Jr. (a.k.a. Shadow Mario) with his Magic Paintbrush would occasionally contain traces of lava. This lava, however, did not result in immediate death.
In the Mario Kart series, lava appears in many race courses. Falling into lava delays the racer, as they must wait for a Lakitu to pull them out.
Donkey Kong Series
Lava is, like on Earth, a naturally occuring and dangerous substance in the Donkey Kong Universe.
Lava's first appearance in a Donkey Kong game was in Donkey Kong Country 2: Diddy's Kong Quest, where it 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 oddly enough, 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.
