Cauldron SIGMA is an activity in Horizon Zero Dawn. It is a HEPHAESTUS facility that manufactures machines for use in terraforming. It is located north of Mother's Heart and situated inside a mountain.
The entrance to SIGMA is a large triangular door, guarded by several Watchers. The interior of the Cauldron is a large, dimly lit maze of chambers and tunnels, with masses of metal and wires that make up the walls, floors, and ceiling. A large amount of automated machinery can be seen working to assemble parts. Aloy must utilize her agility and override tool to navigate the mazes and puzzles of the Cauldron, whilst avoiding or combating the machines roaming inside. The core area is a large circular room where a Fire Bellowback is being assembled inside a force field, guarded by Watchers.
Despite being a machine factory, Aloy can find a number of triangular crates containing vital resources, as well as medicinal fungi. In place of tall grass are vents that can be damaged to create concealing steam clouds. The boss that protects the core is a Fire Bellowback. Once Aloy uses her override on a nearby pillar, the force field will drop and the machine will attack, accompanied by several Watchers. Aloy will have to defeat all machines in the area to access the core and leave.
The following datapoints are obtained automatically during this quest and cannot be missed.