Alien Bases

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

Locations
The Quarantine Enforcement Platform is present on the Mountain Island embedded into the rock face. The purpose of this base was to shoot down "all vessels without proper clearance", preventing other races from coming to the planet, getting infected and spreading the disease when they leave.

The Disease Research Facility is located in the Lost River. Its purpose was to conduct research on live specimens infected with the Carar bacterium. It was destroyed by a Sea Dragon Leviathan, and thus is suspended at a crooked angle in a giant cavern, surrounded by skeletal remains.

The Precursor Thermal Power Generator is found within the Lava Castle, where it produces the power for most of the precursor technology.

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

Precursor Caches are smaller structures found in the Dunes, Northern Blood Kelp Zone, the Sparse Reef and the Lost River. Each requires a purple Precursor Artifact to access. Unlike the bases, they are not essential for story progression, only containing extra lore and Ion Crystals.

Gallery
In Game Screenshots=

Trivia

 * On a Trello card, the Quarantine Enforcement Platform 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 Quarantine Enforcement Platform 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 Containment Facility.