Alien Bases

Precursor Bases are in-development structures found in many different biomes. They were built by the Precursors around one thousand years ago. Four Precursor Bases in total have been implemented, on the Mountain Island the QEP, in the Lava Castle the PTPG, in the Lost River the DRF, and in the Active Lava Zone the PCF.

The Precursor Caches are much smaller structures and can be found in the Sparse Reef, Blood Kelp Zone, Dunes, Lost River, the Mushroom Forest and the Crag Field. In the future, three more will be added, located in the Mountains, the Grand Reef and the Bulb Zone. The Caches are divided into three types: Sanctuaries, Labratories, and Teleporters.

Locations
The Quarantine Enforcement Platform (QEP) 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 (DRF) 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 (PTPG) is found within the Lava Castle, where it produces the power for most of the precursor technology.

The Primary Containment Facility (PCF) can be found 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, the Lost River and the Mushroom Forest. The Dunes, Blood Kelp Zone and Sparse Reef all require a purple Precursor Artifact however, the Lost River Cache requires an Orange Artifact to access. Unlike the bases, they are not essential for story progression, only containing extra lore and Ion Crystals. The Mushroom Forest and the Crag Field Cache currently requires no Artifact but they have Precursor Teleporters that will connect to the Primary Containment Facility. In the future, the Mountains, the Grand Reef and the Bulb Zone will also have Teleporter Caches.

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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.
 * The Primary Containment Facility is not accessible at this time.