The Cyclopes are giants who are really good at building and making stuff. They live under the ocean. Chiron tells us, "the forges of the Cyclopses are under the ocean which gives Poseidon some influence over the makers of his brother's lightning. Zeus believes Poseidon has taken the master bolt, and is now secretly having the Cyclopses build an arsenal of illegal copies, which might be used to topple Zeus from his throne" (9.89). They, apparently, built Olympus for the Olympians.