Lighthouse Reef is a part of the atoll’s oval reef structure and home to a nesting colony of endangered, rare red-footed boobies. Lighthouse Reef provides scenic underwater topology and offers excellent, varied diving. A fascinating phenomenon at Lighthouse Reef is the “Blue Hole”. A mammoth-size cave, once dry as evidenced by stalactites, has been submerged since the Ice Age. A portion of its ceiling collapsed at some time, forming a blue hole more than 400 feet deep and nearly 1,000 feet in diameter.