Underwater Forges of the Cyclopes

"He's a Cyclopes. They work the forges of the gods. They have to be immune to fire. That’s what I was trying to tell you.”- Annabeth Chase, The Sea Of Monsters.

The Underwater Forges Of The Cyclopes
In The Sea Of Monsters, according to Annabeth, Cyclopes work in the underwater forges of the gods. These forges are located underwater in Atlantis(Poseidon's Kingdom). Not much is known about its setting except that the workers have to work in volcanic conditions.

The Cyclopes had apparently always worked with Hephaestus.