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 will be 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.

There will be a Precursor Base i nside the  Lava Castle, but 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 rooms.

Future Plans

 * The "gun", as well as other Precursor Bases, will not be revealed until October 2016. The door to the "gun" will also be locked.
 * Some Precursor Bases will have a Precursor variant of a Moonpool.

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".