Alien Bases

Alien Bases are ancient structures found in many different biomes. They were built by the Precursors around one thousand years before the Aurora crashed. Four Alien Bases exist in total, on the Mountain Island the Quarantine Enforcement Platform, in the Lava Castle the Alien Thermal Plant, in the Lost River the Disease Research Facility, and in the Lava Lakes the Primary Containment Facility. Alien Data Terminals can be found within each, explaining  the functions and purposes of the bases. While there are optional Alien Caches scattered throughout the world, these Bases are vital to the story and must be visited.

Locations
The Quarantine Enforcement Platform is present on the Mountain Island embedded into the rock face. The purpose of this base was to destroy "all vessels without proper clearance", preventing other races from coming to the planet, getting infected and spreading the bacterium 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 Kharaa 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 Alien Thermal Plant is found inside the Lava Castle, where it produces the power for most of the Precursor technology.

The Primary Containment Facility can be found in the Lava Lakes, and is where the Sea Emperor Leviathan is encountered. This Alien Base also features arch rooms, linked to four Alien Arch Caches across Planet 4546B, the positions from left to right as you enter the Primary Containment Facility are: Bulb Zone, Mushroom Forest, Lost River, and Crag Field.

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Trivia

 * 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".