The Subterraneans were creatures that lived below the surface of the Earth.

In 1950, Mestizer commanded the Subterraneans to kill several individuals across London, setting their King free. Mestizer was killed by the King as it broke free from the London Underground, but after Honoré Lechasseur and Emily Blandish changed history to stop the King, the events leading to Mestizer's death did not occur. (PROSE: The Tunnel at the End of the Light) Later Emily compared another ritual to Mestizer's attempts to summon the King with the Subterraneans. (PROSE: The Severed Man)