Alien Bases

Precursor Bases are in-development structures. They were built by the Precursor race around one thousand years ago, and are located within the Mountain Island, Lava Castle and the Lost River. One more Precursor Base is planned to be implemented in the Active Lava Zone.

Locations
(*) for unimplemented

A huge tower built by the  Precursors  is present on the Mountain Island. Atop this tower is the Precursor Ground-To-Air Weapon System, the purpose of which is to shoot down "all vessels without proper clearance".

The Secondary Bio-Research Facility is located in the Lost River. It's purpose was to conduct research on the Carar bacterium. It was destroyed by an escaped Sea Dragon Leviathan, and is thus suspended at a crooked angle in a giant unlit cavern.

The Precursor Thermal Power Generator can be found i nside the  Lava Castle, it powers the majority of the precursor technology. Currently the base is bare, and has only one room.

The Primary Research Facility will be located in the Active Lava Zone, and is where the Sea Emperor will be encountered. This Precursor Base will also feature teleporter rooms. (*)

Future Plans

 * The Precursors might not be seen in-game, but their technology will be present.

Trivia

 * On a Trello card the Precursor "Gun" was referred to as the Precursor G.U.N, or "Precursor Global Defense System". This is a developer joke as "guns are not allowed in Subnautica" yet the model's name is Precursor Gun.
 * Models for the precursor gun and various parts of bases were accidentally revealed during an experimental patch that added them into the entity gallery. A user on the forums revealed screenshots of the bases and they spread quickly. After seeing this, the developers quickly removed them and asked pictures of them to be prevented from spreading; they called it "a huge screw up".