Dark Land

Dark Land is Bowser's kingdom, and the Koopa Troop's main base of operations during the events of Super Mario Bros. 3. A stark contrast to the rest of the Mushroom World, Dark Land is a wasteland-inspired area untouched by sunlight, filled with skulls and flames. It is shown to be south of Ice Land and to the north of Sky Land in Super Mario Advance 4: Super Mario Bros. 3.

Other magmatic lands under Bowser's rule have appeared in other games, but their relation to Dark Land, if any, is unknown.

Super Mario Bros. 3
Dark Land (alternatively Castle of Koopa or Bowser's Castle) first appears as the eighth and final world in Super Mario Bros. 3 and its remake. Large numbers of tanks and airships patrol these lands, which most levels take place on, with only two standard Action Scenes in the entire world. Additionally, one of the areas in the map is so dark that it appears to be completely black, except for a circle of light around Mario or Luigi. It is shown to be south of Ice Land and to the north of Sky Land in Super Mario Advance 4: Super Mario Bros. 3. This is the only world in the game that does not have any Toad Houses, Hammer Brothers patrolling the map, or Spade Panels in it.

It is possible for the player to enter Dark Land via Warp Zone (notably, both Magic Whistles from Grass Land can be used to reach Dark Land without restoring any of the kings).

The Adventures of Super Mario Bros. 3
As it is home to Bowser and the Koopalings, Dark Land appears in quite a few episodes of The Adventures of Super Mario Bros. 3, where it is depicted as a gloomy wasteland. Lava-filled caverns are shown to be beneath the surface of Dark Land in a few episodes as well. However, most of what goes on within Dark Land happens either near or inside Kastle Koopa.

The episodes in which this world is shown as the title card are "Never Koop a Koopa", "Do the Koopa", "Crimes R Us", "Recycled Koopa" and "Super Koopa".

Not counting episodes in which all the Dark Land scenes were restricted to events within the perimeters of Kastle Koopa, Dark Land appears in the following episodes:
 * Reptiles in the Rose Garden: In order to give Kootie Pie rulership of the United States of America as a birthday present, Bowser transports the White House, along with everyone inside, to Dark Land. Bowser later sinks the White House to the bottom of the Mushroom Sea. However, Mario managed to raise the White House to the surface, and with Luigi's help, was able to return the building to its proper spot to Washington, D.C.
 * Mind Your Mummy Mommy, Mario: Luigi, Princess Toadstool and Toad travel through Dark Land via a Steam Cloud Balloon to recover Prince Mushroomkhamen's stolen sarcophagus, only for it to get flung into a tower of Kastle Koopa by a Mushroom Kingdom twister. Luigi then uses the remnants of the Steam Cloud Balloon to construct a steam powered mummy sled, which he, Princess Toadstool and Toad use to escape from Kastle Koopa, and subsequently, Dark Land.
 * Princess Toadstool for President: After discovering that Bowser in ahead in the polls for leadership over the Mushroom Kingdom only by using a Nice Guy Potion, Mario and Luigi travel to Dark Land in order stop Kooky and Cheatsy from making another dose before the election. Upon arriving in Dark Land, Mario and Luigi are pursued by Rocky Wrenches in tanks, which chase them through a pipe into a lava-filled cavern. From that point, Mario and Luigi then have to deal with Fire Chomps, Hot Foots and a Fire Snake before making it to Kastle Koopa.
 * Never Koop a Koopa: When Bowser announces that he is giving up his evil ways, he donates Kastle Koopa to Princess Toadstool. Deciding she wants to turn Kastle Koopa into a retirement home for elderly Mushrooms, Princess Toadstool travels to Dark Land with Mario to check the place out. However, Luigi is wary of the whole thing. He and Toad travel to Dark Land after Mario and Princess Toadstool, where they wait in a boat on the Mushroom Sea for their friends to return from Kastle Koopa. When Mario and Princess Toadstool fail to come out after waiting for a long time, Luigi drags Toad into the Mushroom Sea, and both enter Kastle Koopa via a drainpipe. Discovering that the whole ordeal turned out to be a trap, the group of four reunite and later escape from Kastle Koopa after obtaining Raccoon Power.
 * Do the Koopa: Princess Toadstool discovers a map leading to the Doom Dancer Music Box, which is hidden in the underground Temple of Gloom. Mario, Luigi and Toad accompany Princess Toadstool to Dark Land, but they are immediately spotted by a Dry Bones, who heads to Kastle Koopa to warn Bowser of their arrival. As Bowser is sick, he orders Bully, Big Mouth, and Cheatsy to follow the Dry Bones, who leads them to where the heroes are traveling. Mario's group brave the cavern's many traps and ultimately make it to the Temple of Gloom, where they succeed in obtaining the Doom Dancer. However, a trap set up by Bully causes Mario to lose the Doom Dancer, which winds up in Bully's hands. Bully then brings uses the Doom Dancer's power to bring everyone back to Kastle Koopa, where Bowser, who is unable to hear well due to his cold, takes the Doom Dancer from Bully. Bowser then uses the Doom Dancer to force everyone in the Mushroom World to dance forever, but the Mario Bros. foil his plan.
 * Recycled Koopa: The Koopas are ordered to dump all their garbage, and the place they do so is in Princess Toadstool's Castle. Mario, Luigi, Princess Toadstool and Toad manage to steal one of the Koopa Kids' wands to remove all the garbage and send it back where it came from. The heroes believe the Koopas will try again and spy on them with Luigi's Plumber's Periscope, and see the Koopalings dumping garbage down another pipe leading to the Real World.
 * The Venice Menace: Although Dark Land is unseen in this episode, Bowser's intention is to scare all the humans out of Venice so that he can warp the entire city to Dark Land and turn it into a water park for Kootie Pie.

Nintendo Adventure Books
Dark Land is the setting of the fourth Nintendo Adventure Book, Koopa Capers, where Bowser has a magic carpet bring Luigi to the kingdom to help (whether he wants to or not) in the search for Wendy O. Koopa after the Koopaling disappears. The two main areas of the region that can be explored in the book are a gloomy and trap-filled fortress, and a highly volcanic area known as the Magma Pits.

In the proceeding book, Pipe Down!, Dark Land (though here referred to as Dark World) is mentioned once by Ludwig von Koopa, who states that he intends to take his new ballet, Dribbling Beauty, on a seven-year tour of the kingdom.

The Super Mario Bros. Movie
In The Super Mario Bros. Movie, after Mario and Luigi are accidentally separated from each other while traveling through a Warp Pipe, Luigi ends up in the Darklands, where he eventually finds himself surrounded by many Dry Bones. Luigi reaches a fortress that is surrounded by a lava moat and gets inside, which appears abandoned at first, and is captured by the Shy Guys.

Trivia

 * According to Lemmy in the Japanese instruction manual, the kingdom's activity was top secret even to the Koopalings, as he claims Bowser was exclusively in charge.
 * The background music for Dark Land was covered for the minigames Skateboard Scamper and Piranha's Pursuit, from Mario Party.
 * Dark Land's background music in the NES version was sampled in the song "King Bowser" in Super Smash Bros. Ultimate, which is an arrangement of Bowser's battle theme.
 * Dark Land's music can also be heard in Bowser Castle in Mario Kart DS.