Great Blue Hole
The Great Blue Hole is a giant marine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll from the mainland and Belize City. The hole is circular in shape, across and deep. It was formed during several episodes of quaternary glaciation when sea levels were much lower. Analysis of stalactites found in the Great Blue Hole shows that formation took place 153,000; 66,000; 60,000; and 15,000 years ago. As the ocean began to rise again, the cave was flooded. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System, a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).
Schuylkill Gap
Schuylkill Gap is a water gap through Blue Mountain located about 3 miles north of Hamburg, Pennsylvania. The borough of Port Clinton, Pennsylvania resides within the gap itself.
Formed in much the same way as Pennsylvania's other water gaps, Schuylkill Gap is near Hawk Mountain, Weiser State Forest, and some State Game Lands. PA Route 61 as well as the Schuylkill River pass through the gap.