Underground seas are areas where an entire ocean or sea exists under the earth. These underground seas will feature elements of both normal seas (or lakes if freshwater) and subterranean worlds. Underground seas may be inhabited by subterranean species or people. They may contain strange aquatic creatures, either long since thought extinct or never seen before on the surface. If inhabited by humans or humanoids, these seas may contain lighthouses, raft towns, shipwreck towns, and other fantasy world sea elements.

Underground seas feature elements of both oceans and cavern systems. They may be attached to the surface via sea caves or have openings high above like cenotes. They may have shores that lead to normal cavern systems. They may be fed by underground rivers. Like underground lakes, these seas may exist within a single vast cavern or be spread across multiple caverns, some of which may be entirely flooded.

Navigation across underground seas is an area where combinatorial worldbuilding can provide new elements. While the stars aid in navigation on normal seas, perhaps luminescent material high on the chamber ceiling act like constellations for underground seas. As with all underground areas, how the space may be illuminated is an issue that should be addressed by a worldbuilder. Caves in fantasy worlds frequently contain glowing mushrooms, crystals, or minerals. Underground seas may feature these elements as well as bioluminescent algae, seaweed, or other aquatic elements.