The Enclave Oil Rig, the primary self sufficient fortress of the pre war heads of the U.S. government and top tier CEOs and previously known as the Poseidon Oil Rig, was built over the last known supply of crude oil thousands of feet beneath the Pacific after sabotaging Chinese efforts to take the area.
It is led by President Richardson, the enclave's elected president of March 2220 and under his order seeks to release a modified strain of FEV into the airstream to kill each and every soul living outside of their vaults. In this effort, it sent soldiers to the buried and once Master controlled Mariposa Military Base to uncover the newest strain of FEV.
One man who was sent to the area, Frank Horrigan was exposed to the extremely potent substance, and turned into a super mutant who miraculously retained his mental capacity. The Enclave used this to their advantage to fit the man in a suit of power armor, and turning him into the most deadly and powerful enemy of the fallout series, second in power only to Liberty Prime.
Meanwhile, the Enclave also worked on upgrading their power armor (only succeeding after many, many failed prototypes) and other technologies. It housed the absolute best facilities in the post nuclear land as well as enough Vertibirds to initiate guerrilla attacks on any location in the U.S. within a matter of minutes.
Unfortunately for them, their attack on the Chosen One's tribe and Vault 13, led to their demise as the Chosen One repairs a ship docked in San Francisco, makes his way to the oil rig, and overloads its cooling systems to create a thermonuclear explosion crippling the pre war republic of the U.S.