The discovery in the Gulf of Mexico was announced last month by the U.S. Geological Survey.
It was tentatively identified as a coral reef in 1999 by a team from the University of South Florida. But it took several more years of research to confirm it as a living reef that depends on light filtering down from the surface.
"We were all blown away by this bizarre, flat, living sea floor covered with blue and brown corals and lettuce-like green algae," researcher Bret Jarrett said of seeing live video from an unmanned submersible.
The video revealed a stunning number of fish, both deep and shallow water species: giant red grouper, scamp, damselfish, angelfish, rock beauty, hogfish and bass.
The reef is on Pulley Ridge, a vast area west of the Dry Tortugas, a cluster of seven islands 70 miles west of Key West. The reef is up to three miles wide and runs for about 20 miles.
Shallow-water reefs tend to grow vertically, like those off the Florida Keys. Pulley Ridge coral grows flat because it has adapted to the low light.
"Corals require light to grow, and so they spread out laterally as opposed to vertically," Jarrett said. "They've adapted to the situation, they've maximized the sunlight."
Officials who oversee the gulf are now wondering how to preserve the reef.
The scientists' research has been presented to the Gulf of Mexico Fishery Management Council, which will decide in the coming months whether to restrict fishing or trawling in the area.