“Million Mounds” may be overstating the case a bit, but there’s no doubt it’s one of the most extensive deep-water coral reefs on the planet. Or make that part of one. Scientists recently discovered that the system extends far beyond Million Mounds—the biggest deep-water coral reef yet seen.

The entire complex stretches along the southeastern Atlantic coast of the United States. It’s a few dozen miles out, from Miami to near Charleston. It encompasses about 50,000 square miles, at depths of about 2,000 feet or greater.

Million Mounds had been the only part of the system that had been studied in detail. Most of the corals are on the many mounds and ridges found across the region—hence the name “Million Mounds.”

Scientists used ships on the surface, plus robotic submersibles, to map a much larger region. The surface vessels scanned the ocean floor with sonar. And the submersibles provided close-up looks at selected locations.

The corals aren’t like the vibrantly colored ones found in shallower seas. Instead, they’re all white. That’s because they’re mainly the “stony” part of a coral. They don’t contain the same microorganisms that provide the color for their shallower cousins. Those organisms need sunlight, and it’s too dark for them in the deep ocean.

The deep-water coral filter food from the water—bits of organic matter that drift to the bottom. That allows them to survive—a lot of them—in the deep waters off the American coast.