Alien Bases

Precursor Bases are in-development structures. They were built by the Precursor race around one thousand years ago, and one is currently located at the Mountain Island. Three more Precursor Bases are planned to be implemented in the Lava Castle, the Lost River, and 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 will be located in the Lost River. Its purpose was to conduct research on the Carar bacterium. It was destroyed by an escaped Sea Dragon Leviathan, and thus well be suspended at a crooked angle in a giant cavern. (*)

The Precursor Thermal Power Generator will be found within the   Lava Castle, it powers the majority of the precursor technology. (*)

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".