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. Alien Data Terminals can be found within each, explaining about the functions and purposes of the bases.

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 Leviathan is encountered. This Precursor Base also features teleporter rooms, linked to the Teleporter Caches across Planet 4546B.

Gallery
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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".