Subterranea is a side-scrolling shooter game for the Atari 2600, where your objective is to find treasure crystals hidden within the Hexuplex Caves. You control a cave ranger, navigating through the caves and shooting various types of aerobots guarding the area. Once all the aerobots on a cave level are destroyed, a passageway opens up, allowing you to delve deeper into the caverns.
When you reach the final level of the cavern, you can collect the treasure crystal and be transported to the next, more challenging set of caves to find another crystal. Beware of deadly aerobots and avoid crashing into cave walls, as these will quickly drain your energy, especially if you hit a skull. When all your reserve cave rangers have crashed, the game is over.
As you progress through the caves, they become narrower, and the enemies move faster. Subterranea offers both standard and advanced skill levels for players to choose from.