Alien Bases

Precursor Bases are in-development structures. They were built by the Precursors around one thousand years ago. Four Precursor Bases are planned to be implemented on the Mountain Island, in the Lava Castle, the Lost River, and the Active Lava Zone.

Locations
A huge base built by the Precursors is present on the Mountain Island embedded into the rock face. Atop this base 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. Its purpose was to conduct research on the Carar bacterium. It was destroyed by an escaped Sea Dragon Leviathan, and thus is suspended at a crooked angle in a giant cavern.

The Precursor Thermal Power Generator is found within the Lava Castle, where it powers the majority of the precursor technology. (Barely implemented) (*) 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. (*)

(*) for unimplemented contents

Gallery
In Game Screenshots=

Trivia

 * On a Trello card, the Precursor Ground-To-Air Weapon System 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 Ground-To-Air Weapon System 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".
 * Precursor Teleporters will be present in each major biome and at important locations - they will all link to the Primary Research Facility.