"Lego Underground" is not the official name of any specific LEGO theme. However, LEGO has produced several popular themes and sets that take place in subterranean, mining, or caving environments.
Themes Focusing on Underground Exploration/Mining
LEGO Rock Raiders (1999-2000, Discontinued): A popular classic theme where a team of humans accidentally lands their spaceship in a deep underground cavern. They use specialized vehicles to mine energy crystals while encountering rock and ice monsters.
Notable Set: Rock Raider HQ (set 4990) was the main base set with light and sound features.
LEGO Power Miners (2009-2010, Discontinued): The spiritual successor to Rock Raiders. A new team of miners battled various types of rock monsters, including lava and ice monsters, who were trying to cause earthquakes.
Notable Set: Crystal King (set 8708) and Rock Wrecker (set 8707) were popular sets in this theme.
LEGO Adventurers: Dino Island (2000, Discontinued): While primarily on an island, several sets involved cave systems and a large research compound, as the team searched for dinosaurs and artifacts.
Modern Themes with Underground Elements
LEGO Minecraft (Current): A significant portion of the game and the physical sets are dedicated to mining, caving, and exploring underground biomes like "The Cave" and "The Nether."
Notable Set: The Abandoned Mine (set 21166) is a good example of a cave-focused set.
LEGO Ninjago (Current): The Ninjas often have adventures in vast underground environments and cave systems, sometimes involving a "Digiverse" or the "Underworld" where they fight various villains.
LEGO City (Current): Modern City sets, particularly those within the Space, Arctic, or Jungle sub-themes, sometimes include elements of cave exploration or underground bases. The 2024 Jungle theme features cave exploration as a key play feature.
LEGO Education (Current): Products designed for STEM learning sometimes feature builds related to cave systems or drilling, teaching principles of mining and geology.
Fan Creations (MOCs)
Within the LEGO fan community, "underground" is a very popular theme for MOCs (My Own Creations). Builders frequently create elaborate "caves," "mines," or "subways" using specific building techniques to make rock walls and use light bricks to simulate mining lamps or lava.