The origin of Blue Hole dates back to an ice age about 15,000 years ago. Enough sea water was frozen in glaciers during this time to lower sea level more than 350 ft, exposing the limestones of Lighthouse Reef. Huge subterranean caverns formed when fresh water flowed through the limestone deposits. Since then, the roof of the cavern has collapsed to form the sinkhole.
Made famous by a Jacques Cousteau's 1970 Calypso expedition, Blue Hole is one of the best known dive sites in Belize. It is a circular, deep depression in the center of more than 75 sq. miles of shallow, blue-green water. Its diameter at the rim measures 1,045 ft, whereas its maximum depth is 412 ft. Except for two narrow passages on the eastern and northern rims, Blue Hole is completely rimmed by living coral.
For the advanced diver this site is well worth the trip. You should plan to dive either the north or south side to a depth of 100-150 ft where the shallowest cave features are found. Begin your dive by snorkeling to the coral rim. This serves two purposes: first, it conserves air, and second, it provides an opportunity to get everyone making the dive together before you descend. Your no-decompression bottom time is short at the planned depths so it is best to snorkel toward the center to Blue Hole, just beyond the vertical wall, before descending.