Quarantine Enforcement Platform

The Precursor Ground-To-Air Weapon System is a huge gun located by the Mountain Island. It created the energy pulses responsible for causing both the Degasi and Aurora to crash on Planet 4546B, as well as destroying the Sunbeam.

The weapon was built by the Precursor race around one thousand years ago, in order to prevent the spread of Carar.

The door leading to the control panel of the system is protected by a scanner that prevents life forms infected with Carar from entering, the player must cure himself of Carar before being able to disable the gun and leave the planet.

Ion Crystals will be found here.

Interior
There are two entrances to the Precursor Ground-To-Air Weapon System. One is located underwater at the bottom, resembling a hanger, and a smaller entrance on the island. Inside the underwater entrance there is a large room, with a door blocked by a force-field. In the upper entrance there are unusual structures and a cube with a log icon, the PDA will translate its contents. There is also a strange reactor-like green object.

Trivia

 * The in-development name for the Precursor Ground-To-Air Weapon System was the Precursor G.U.N, or Precursor Global Defense System. This is a running joke, as "guns are not allowed in Subnautica".
 * The G.U.N was leaked when developer's left it in entity-gallery, quickly after the update with the gun dropped, a forum user found the Precursor Ground-To-Air Weapon System, and many other things.