Fictional subterranea

fictional country in Middle-earth

setting for Lewis Carroll's Alice's Adventures in Wonderland

The Underdark is a fictional setting which has appeared in Dungeons & Dragons role-playing campaigns and Dungeons & Dragons-based fiction books, including the Legend of Drizzt series by R. A. Salvatore. It is described as a vast subterranean network of interconnected caverns and tunnels, stretching beneath entire continents and forming an underworld for surface settings. Polygon called it "one of D&Ds most well-known realms".

fictional underground city in The Matrix