Lava

Lava is, quite simply, molten rock, just like in real life. It can be found as a hazard in many parts of Mushroom World, especially inside castles.

When Mario touches lava, it can scorch him terribly, causing him to run around, almost uncontrollably, in pain. In some instances, it may result in him losing some health (as seen in Super Mario 64, Super Mario Galaxy and its sequel and Paper Mario), or instant death (as seen from Super Mario 3D Land, Super Mario Sunshine in Corona Mountain and most 2D platformer Mario games, such as Super Mario Bros. and Super Mario World). Even a Star will not protect Mario from this deadly obstacle. The effect lava has on Bowser varies as well, ranging from not affecting him at all to actually killing him at least once. This could mean that there are different temperatures (or even separate types) of lava that differ from game to game.

Super Mario Bros.
Lava first debuts in Super Mario Bros., where it appears in the fourth level of each world. If Mario or Luigi falls into the lava, they will instantly lose a life. At the end of the level, Mario or Luigi must destroy the bridge that Bowser is standing on, causing him to fall into the lava.

Due to the graphical limitations of the NES at the time, the "lava" is really red-colored water, and is only a background object (proven by characters falling through the lava and not dying the moment they touch it).

Super Mario Bros.: The Lost Levels
Lava reappeared in Super Mario Bros.: The Lost Levels and had the same appearance and purpose as in Super Mario Bros.

Super Mario Bros. 3
In Super Mario Bros. 3, lava occasionally appears in the Fortress levels in some worlds. Falling into the lava will cause Mario or Luigi to instantly lose a life. In some levels, lava pools can also be placed upside down, on ceilings.

Super Mario World
In Super Mario World, lava appears in the fortresses and castles level. Once again, Mario or Luigi will instantly lose a life should they fall into the lava.

Super Mario 64
In Super Mario 64, lava appeared in Lethal Lava Land and Bowser in the Fire Sea, and covered much of the stage. This lava would bubble and release Podoboos and Fire Snake-like clumps of fire. When Mario touched lava, he would jump into the air with smoke coming out of him, and he would quickly lose three wedges of health. In his Capless Form, Mario would lose four wedges instead, meaning that in his capless form, Mario will lose a life in just two hits from the lava. Both lava stages featured enemies called Bullies that would try to knock Mario into lava.

Super Mario Sunshine
In Super Mario Sunshine, there are two different kinds of lava. The first type is found only in Corona Mountain. It covered the entire bottom of the stage, and served as a major stage hazard in that level. If Mario touched the lava in Corona Mountain, he would instantly lose a life. The second type is the Lava Goop painted by Bowser Jr. (a.k.a. Shadow Mario) with his Magic Paintbrush. This lava did not result in immediate death, but Mario would leap into the air uncontrollably, covering little horizontal distance. Therefore, if there is a lot of the lava goop when Mario touches it, death may still be inevitable.

Super Mario Galaxy
In Super Mario Galaxy, lava is found mainly in each Bowser and Bowser Jr. level, where it comprises most of the landscape (with a few exceptions of course). In fact, Bowser Jr.'s third level in Super Mario Galaxy is actually called Bowser Jr.'s Lava Reactor, and features considerably more lava than the other Bowser or Bowser Jr. levels. Other galaxies, while not entirely comprised of only lava, do rely heavily on the presence of lava as a major obstacle in some areas. Like Super Mario 64, when Mario touched the lava, he would catch fire and jump into the air uncontrollably, losing 1 wedge of health.

Super Mario Galaxy 2
In Super Mario Galaxy 2, the same applies for Bowser's first fortress in the game, Bowser's Lava Lair. Some galaxies between the two games consist entirely of lava-based obstacles, including the Melty Molten Galaxy and the Melty Monster Galaxy.

New Super Mario Bros.
In New Super Mario Bros., lava appears in the most Castle levels and few levels in World 8. The lava also rises on some levels. As like with the other 2D platformer Mario games, when Mario or Luigi falls on the lava, he would instantly die.

Super Mario 3D Land
In Super Mario 3D Land, lava appears in Castle levels, some levels of World 8 and few levels in Special Worlds. Unlike some other 3D Mario games, if the player touches lava in this game, Mario or Luigi will instantly lose a life just like in 2D Mario games and Super Mario Sunshine.

Mario Kart series
In the Mario Kart series, lava appears in many race courses, most frequently Bowser's Castle. Falling into lava delays the racers, as they must wait for a Lakitu to pull them out to continue the race.

Paper Mario series
Despite lava appearing in almost every Mario game (or almost every video game for that matter), Lava is rarely seen in the Paper Mario series. In Paper Mario there are only two areas to feature lava; the first is inside the volcano Mt. Lavalava, and the second is inside Bowser's Castle. In this game, if Mario falls into the lava, he loses five hit points, and on most occasions, he needs to have Parakarry carry him over it or have Lakilester float over it. Lava appears again in Paper Mario: The Thousand-Year Door. In this game, it is only found during the two Bowser events; the first around Bowser's Castle, and the second in a stage reminiscent of Super Mario Bros.. Mario himself never encounters any lava at all in this game. In Super Paper Mario, there is no lava at all, save the lava around Bowser's Castle in the introduction screen.

Mario Party series
In the Mario Party series, lava also appears, acting as a hazard to be avoided primarily in mini-games.

Most Bowser-related game boards in the series involve lava in some way.

Mario Party 2
The minigame Hexagon Heat from Mario Party 2 involves moving over to the colored spot Toad indicates using the flags he holds. The platforms will slowly fall into lava as a different one is called. Another minigame in both the original and the sequel, Handcar Havoc, involves steering a handcar, and leaning so it does not tip over the edge of the tracks and into lava. Lava Tile Isle involves pushing players off the sinking tiles and into lava.

Mario Party 4
The minigame Chain Chomp Fever in Mario Party 4 involves staying out of the way of the Chain Chomp, while also avoiding falling into the lava. The mini-game The Final Battle! involved getting to and fighting Bowser by solving puzzles and, once again, avoiding falling into lava.

Mario Party 6
The Mario Party 6 minigame Burnstile involves leaping over a turnstile to avoid it, while also trying to stay on their team's area, which is surrounded by lava.

Mario Party 7
Despite not having lava as a hazard, the minigames Treasure Dome and Tunnel of Lava! from Mario Party 7 is filled with it. The minigame Magmagical Journey! contains much lava that acts as an obstacle.

Mario Party 8
The minigame Lava or Leave 'Em from Mario Party 8 once again involves dodging Podoboos while staying away from lava. Lava also appears in the mini-game Lava Lobbers.

Mario Party 9
In Mario Party 9, lava appears in Magma Mine, as one of the most dangerous obstacles in the board, taking half of the captain's Mini Stars when he or she touches it. In Magma Mayhem, the players must knock their adversaries out of a tilting platform, making them sink into the magma. It also appears in the minigames Tumble Temple, Bowser Pop and Double Pounder, as an aesthetic element. In the Boss Battle Spike Strike, the players and Spike are inside a volcano, on a rock surrounded by lava. In the second half, the lava rises, giving less time to the players think. In Chain Chomp Romp, the volcanoes in the background are in eruption, and lava is seen running down them.

Mario Party Advance
In Mario Party Advance, the mini-game Stompbot XL involves trying to avoid the lava heading towards the player, while avoiding obstacles like walls, rocks, and small patches of lava. Also, the mini-game Go-Go Pogo has the player using a pogo to get to the end while avoiding the lava below and other obstacles.

Donkey Kong series
Lava is, like on Earth, a naturally occurring 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, swam through after Krematoa's ancient volcano is re-activated.

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

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

Trivia

 * Contrary to popular belief, using a Fire Flower to transform into Fire Mario does not make Mario invincible to lava, and will damage him normally if entered.
 * On the other hand, collecting an Ice Flower will allow Ice Mario to skate across the surface of lava filled areas unscathed (only in Super Mario Galaxy, though).
 * Even though lava doesn't appear in Super Paper Mario, the tattle for the Lava Bubble says that the player must avoid them while jumping over lava.