Cauldron

A Cauldron is an underground facility in Horizon Zero Dawn where machines are built. By overriding their cores, Aloy unlocks the ability to override and control machines that are built in those Cauldrons. There are a total of four major Cauldron activities in Horizon Zero Dawn, plus a fifth Cauldron in Horizon Zero Dawn: The Frozen Wilds.

History
Project Zero Dawn envisioned a series of underground facilities to rebuild a lifeless Earth. These facilities would be controlled by GAIA while the subfunction HEPHAESTUS would oversee construction of Cauldrons where specialized machines would be designed and constructed for use in terraforming. The inhabitants of the New World are aware of the cauldrons, and some have deduced that they are where machines are built. A few have attempted to venture inside and discover its mysteries, although only one has been successful.

Cauldrons
There are a total of five physical Cauldrons in the Horizon Zero Dawn world. A sixth Cauldron, Cauldron PSI, is known to exist, although its location remains unknown, since Aloy has the PSI Overrides when she unlocks the Override ability.
 * Cauldron SIGMA - Located east of Daytower and west of Devil's Thirst. Features basic puzzles and a Fire Bellowback boss fight.
 * Cauldron RHO - Located east of Meridian and north of the Jewel. Features moderate puzzles and a Snapmaw and Ravager boss fight.
 * Cauldron XI - Located south of Meridian inside the Jewel. Features an Eclipse camp and several machines and cultists for a boss fight.
 * Cauldron ZETA - Located east of Maker's End and north of Cut-Cliffs. Features no puzzles and a Thunderjaw boss fight.
 * Cauldron EPSILON - Located in The Cut, east of Firebreak inside Thunder's Drum. Features moderate puzzles and a Fireclaw boss fight.

Trivia

 * Cauldrons were designed to be unnatural and alien in appearance.
 * Each Cauldron is named after a Greek letter.