Mount Justice, or the Secret Sanctuary, is a secret based under a mountain located in the town of Happy Harbor, Rhode Island. It was used as a base for the Super Friends and the Justice League of America, before it was compromised and they moved to the Hall of Justice.
When Cyborg formed Young Justice, he moved his team there (along with Red Tornado's AI). When the Justice League was re-established and Titans Tower reopened, Cyborg took his team there and they became the third generation of Titans.
But when Tim Drake decided to make his own team, to take do black-ops missions that the Justice League couldn't do, he brought the former members of Young Justice and formed The Team, taking back Mt. Justice as their headquarter.