Alien Bases

Precursor Bases are in-development stuctures. They were built by the Precursor race, and are to be located within the Mountain Island, Lava Castle, Active Lava Zone, and the Lost River.

Locations
By the Mountain Island a huge tower built by the Precursors will be present, atop this tower is a large "gun".

There will be a Precursor Base located in the Lost River, it will be below the Precursor Gun and may be connected. Its purpose is currently unknown.

Inside the Lava Castle there will be a Precursor Base, its purpose is currently unknown.

The final Precursor Base will be located in the Active Lava Zone, the area is known as the "Emperor Facility" and is presumably where the Sea Emperor will be encountered. This Precursor Base will also feature teleporter and moonpool rooms.

Future Plans
The "Cannon" will not be revealed until September 2016. The door to the "cannon" will also be locked.

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 some of the Precursor 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".