Underwater caves (submarine caverns) are cavern systems found underneath a body of water. They may be found underneath an ocean or sea, beneath a lake, or within a reservoir. Like regular caverns, submarine caves may contain a network of chambers. In fictional worlds, underwater caves may contain large pockets of breathable air trapped within the cavern system. There may be shipwrecks, treasure, lost underground civilizations, underground seas, and other hidden elements contained within these isolated chambers. These caves may be home to deep sea monsters, leviathans, and lost scuba divers.

When designing underwater caves, consider the size of the chambers. A narrow cave opening may open into a vast and dark chamber of open water, or may lead to a labyrinthian maze of flooded tunnels.

Cave formations like stalactites, stalagmites, and columns, which are formed by dripping water, will not be present in underwater caves (where water cannot drip) unless the cave first formed above ground and then became flooded.