When you arrive at the entrance to Cauldron Zeta in Horizon Zero Dawn, you will find that there is a neon blue shield that is blocking Aloy from being able to access the door. When you look at the quest it will say to find the hidden entrance to Cauldron Zeta, where is this entrance?
The hidden entrance to Cauldron Zeta, isnt too hard to find. If you are facing the main entrance, look to the left of the area and you will see a stalker walking around on a small cliff edge. Make your way up here by climbing on a nearby pillar. Kill the stalker and walk to the end of the cliff edge. You will begin to notice a blue glow coming from a small cave at the end. Go inside, this is the hidden entrance to the cauldron.