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The origin of the Blue Hole can be traced back 15,000 years to an earlier ice age. During this time, the glaciers from the north trapped so much water in their frozen expanses that the sea level in these areas was lowered by more than 350 feet. The limestone in the area now known as Lighthouse Reef became exposed in large part. Freshwater began flowing through the limestone deposits and huge, underground caverns formed as a result. In the area of the Blue Hole, the roof of one of these caverns has since collapsed, creating the eerie, circular sinkhole now referred to as the "Great Blue Hole". Viewed from the air, it resembles an almost perfectly circular hole, filled with deep, blue water in marked contrast to the aquamarine waters of the surrounding shallow reef areas. It is an unmistakable landmark in the middle of more than 75 square miles of shallow, atoll waters.