The Blue Hole is a roughly circular submarine sinkhole about 120 m deep, ringed by reef and famous for the Arch, a 26 m tunnel whose ceiling sits at 55 m and opens to the open sea. Most recreational divers explore the rim wall and the shallow saddle, while the deep Arch transit is strictly the domain of trained technical divers. It is one of the most beautiful and most infamous dive sites on Earth, marked by a sobering memorial wall to lost divers.