The Underdark is a vast, subterranean realm found beneath the surface of many fantasy worlds, particularly in the Dungeons & Dragons universe. It is characterized by its dark, labyrinthine tunnels and caverns, inhabited by various creatures, including drow, duergar, and other monstrous beings. The environment is often filled with bioluminescent fungi and underground rivers, creating a unique ecosystem. This underground world is known for its treacherous terrain and complex political structures among its inhabitants. The drow, a race of dark elves, are particularly prominent in the Underdark, often engaging in power struggles and intrigue. The Underdark serves as a setting for many adventures, offering challenges and mysteries for explorers.