Aquarius Reef Base is an underwater habitat in the Florida Keys National Marine Sanctuary near Key Largo and the world’s only undersea research laboratory still operating. Completed in 1986 and later moved to the Florida Keys after service in the U.S. Virgin Islands, it supports marine science through saturation diving, allowing crews to live underwater and work for extended periods studying coral reefs, marine life, and seawater chemistry. The habitat also has served NASA’s NEEMO missions as a spaceflight analogue. Despite hurricane damage, a fatal diving accident, and funding uncertainty, it continues under Florida International University as a rare platform for research, training, and outreach.