A feature attraction of Diving in Belize, Especially for divers with a appreciation of geographical phenomena, is the opportunity to explore the famed Blue Hole. Part of the Lighthouse Reef System, it lies approximately 60 miles off the mainland out of Belize City. It is one of the most astounding dive sites to be found anywhere on earth, right in the center of Lighthouse Reef is a large, almost perfectly circular hole approximately one quarter of a mile (.4 km) across. Inside this hole the water is 480 feet (145 m) deep and it is the depth of water which gives the deep blue color that causes such structures throughout the world to be known as "blue holes."

Like a giant pupil in a sea of turquoise, The Blue Hole is a perfectly circular limestone sinkhole more than 300 feet across and 412 feet deep. The array of bizarre stalactites and limestone formations which mould its walls seem to become more intricate and intense the deeper one dives. Near to The Blue Hole, one of Belize's largest protected areas, Half Moon Caye Natural Monument, encompasses 10,000 acres of the atoll and 15 square miles of surrounding waters. 

The diameter of the circular reef area stretches for about 1,000 feet and provides an ideal habitat for corals to attach and flourish. The coral actually breaks the surface in many sections at low tide. Except for two narrow channels, the reef surrounds the hole. The hole itself is the opening to a system of caves and passageway that penetrate this undersea mountain. In various places, massive limestone stalactites hang down from what was once the ceiling of air-filled caves before the end of the last Ice Age. When the ice melted the sea level rose, flooding the caves.

The temperature in the Blue Hole at 130ft is about 76F with hardly any change throughout the year at that depth.

For all the practical purposes the over 400-foot depth makes the Blue Hole a bottomless pit. The walls are sheer from the surface until a depth of approximately 110 feet where you will begin to encounter stalactite formtions which actually angle back, allowing you to dive underneath monstrous overhangs. Hovering amongst the stalactites, you can't help but feel humbled by the knowledge that the massive formation before you once stood high and dry above the surface of the sea eons ago. The feeling is enhanced by the dizzying effect of nitrogen breathed at depths. The water is motionless and the visibility often approaches 200 feet as you break a very noticeable thermocline. 

SOURCE: Diving in Belize and Ambergris Caye
 

Approximately 60 miles (100 kilometers) from Belize City, the almost perfectly circular Blue Hole is more than 1,000 feet (300 meters) across and some 400 feet (120 meters) deep. The hole is the opening to what was a dry cave system during the Ice Age. When the ice melted and the sea level rose, the caves were flooded, creating what is now a magnet for intrepid divers.

The Great Blue Hole, located near Ambergris Caye, Belize.

The Great Blue Hole is a large underwater sinkhole off of the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 45 miles from the mainland and Belize City. The hole is circular in shape, over 1,000 feet across and 400 feet deep. It was formed as a limestone cave system during the last ice age when sea levels were much lower. As the ocean began to rise again the caves flooded, and the roof collapsed.[1]

On-shore caves of similar formation, as large collapsed sinkholes, are well known in Belize.

This site was made famous by Jacques-Yves Cousteau who declared it one of the top ten scuba diving sites in the world. In 1971, he brought his ship, the Calypso, to the hole to chart its depths.  Investigations by this expedition confirmed the hole's origin as typical Karst limestone formations, formed before rises in sea level in at least four stages, leaving ledges at depths of 70, 160 and 300 feet. Stalactites were retrieved from submerged caves, confirming their previous formation above sea level. Some of these stalactites were also off-vertical by 10° - 13° in a consistent orientation, thus indicating that there had also been some past geological shift and tilting of the underlying plateau, followed by a long period in the current plane.

Wikipedia - Great Blue Hole

National Geographic - Patterns in Nature: Island Aerials

USGS - The Great Blue Hole of Belize

Cambrian Foundation - Blue Hole Origin and Sea Level Change
 

Introduction
By Dr. Jaime Awe
Director, Institute of Archaeology

Caves Branch Cave is one of several subterranean sites that were carved out of the limestone foothills of the Maya Mountains by the very active Caves Branch River. The site provides us with an excellent example of the erosive power of water, and the natural wonders of Belize. Geological research informs us that the cave took several hundred thousand years to form. Archaeological investigations note that the ancient Maya visited the site to conduct important rituals. Today’s modern visitor enjoys a unique opportunity to observe the awesome geological features of the site, and to ponder the nature of prehistoric human utilization.

The Geology of Caves Branch Cave

Millions of years ago, most of the landmass of Belize was covered by a broad, shallow tropical sea. One of the major rock types deposited in this sea was limestone, a rock formed of calcium carbonate. This limestone can have its origin either from biological materials like corals and mollusks, or in some cases the limestone can be precipitated directly from the seawater.

Like the modern Gulf of Mexico, this shallow Cretaceous sea was occasionally subject to violent storms that disturbed the floor of the sea. These storms created a distinctive type of limestone rock called a breccia. Breccia is a rock that is made up of angular pieces of other rocks. In the case of the rock at Caves Branch Cave, the angular pieces of rock are called “rip up clasts.” These are pieces of rock several inches on a side that were torn up and jumbled about before the clasts or pieces had a chance to harden. 

http://www.belize.com/articles/belize-caves-branch.html
 

This is a list of caves of Belize with known depths and lengths.
 

Cave	Depth	Length
Actun A		7000m
Actun Box Ch'iich'	183m
Actun Check		4500m
Actun Lubul Ha		3750m
Actun Nab Nohol inferior		4500m
Actun Tun Kul	160m	18000m
Caves Branch cave
Hitch Tulz		3000m
Petroglyph cave		3000m
St Herman's Cave		3800m

Actun Box Ch'iich'

At -183m, Actun Box Ch'iich' (Cave of the Black Birds) is the deepest cave in the country of Belize. Located above Roaring Creek in the Cayo District, it is one of several caves explored and documented by British and Canadian cavers during a three-week expedition in 1989. The cave contains a small stream and has both an upper and lower entrance, making a through-trip possible.