Bunker SF-Zero-One was a bunker operated by Starfleet located under Starfleet Headquarters. It had duplicate systems for all of Starfleet's command systems. It was built hundreds of years ago, possibly before the founding of the Federation.
In 2375, Captain Jean-Luc Picard led a group of newly graduated ensigns into this bunker when the Breen attacked San Francisco, coordinating the defense of Earth from the bunker. (TNG short story: "Eleven Hours Out")