Anyone who plays Subterranea is going down. Down into the subsurface depths, that is, fending off the attacks of the squid-like Hexuplex and its hordes of Aerobots which track down the player's ship unless the player disposes of them first. After doing away with the tentacled beast's contingent of Aerobots, the player's ship descends further, where it faces a fast-moving, vaguely Vanguard-esque tunnel fight against armed enemies who aren't above using kamikaze tactics to take out the player's ship. The game continues along these lines, with the player attempting to wipe out enough enemies, which opens a portal to go even deeper. If the player manages to survive these chambers, he'll wind up battling another Hexuplex and starting over again at a higher difficulty level.
Like most of Imagic's Atari VCS titles, Subterranea is colorful, but it somehow lacks the "oomph" of earlier classics like Moonsweeper and Atlantis. Subterranea alternates between being too hard and too easy, and is ultimately frustrating as a result.