The Mystery of Blue Holes

The Earth’s oceans are full of mysteries that people seem intent on exploring. Beautiful ocean destinations are popular among tourists. The dangers of the ocean often do not deter scientists and divers from entering risky locations. Many underwater areas are only accessible with proper diving equipment and skills. Blue holes are found all over the globe. These sinkholes house underwater caverns and seawater. Curious divers are often drawn to their mysterious depths. These unique holes offer an interesting structure that varies depending on their formation.


Blue holes present as a mostly circular indention in the earth and have unusually steep walls. Their name comes from the bright blue color of the water that stands out, immensely. The deepest part of the holes is a dark blue. There are usually more shallow areas surrounding blue holes that show a lighter blue. The blue color persists, in part, due to the carbonate sand that makes up the lining of a blue hole. This sand is bright white, and reflects the color back to the top of the blue hole. Many colors on the spectrum enter into the hole, however, the blue is the only one that manages to return.


Scientists theorize that blue holes formed during the ice ages. Sea level during these times may have been around 100-120 meters less than today. Blue holes are often found in areas where limestone is abundant. This type of landscape is sensitive to erosion. Erosion is a part of changing the land across all time periods. Rain and chemical weathering tear away at the land until large holes are formed. This type of erosion ceased once the ice caps melted and filled up the area.

Erosion of a different type takes place in blue holes today. Many blue holes are a melting pot of seawater and freshwater. This causes a reaction that contributes to erosion. The place where saltwater and freshwater meet is called the halocline. This results in a corrosive type of behavior. The rock on the surrounding walls gets eaten away. This is how the famous caverns are formed. They reach out to the sides of the original sinkhole from underneath the water. Not only do some of these begin at great depths; they often continue for a great distance. There is one in the Bahamas that exceeds 2,000 feet in length.

The Karst Process

Karst formations are blue holes that are further from the sea. These inland formations can be quite extensive, resulting in underground caves and rivers. Karst processes are explained as a type of topography that forms when there is a dissolution of sensitive rocks, such as limestone. Dolomite and gypsum are also considered soluble enough to succumb to this erosion. Sinkholes formed from the karst process cause the existence of blue holes.

Oxygen and Bacteria 

Due to poor circulation of the water in blue holes, there is often a lack of oxygen in the deeper levels. This is referred to as “anoxic.” An anoxic environment is not a desirable environment for sea life. Therefore, explorers should not expect to see many living organisms. Bacteria, however, thrives in the environment. Divers should be cautious about their health when diving in blue holes. While life is not supported in blues holes in the present day, there is often evidence of life prior to the rising of sea levels. Fossils of have been found in the interior of blue holes that include both land and marine life. Tortoise and crocodile fossils are among the discoveries.

Famous Blue Holes

There are some very specific areas where blue holes are common. This is due to the existence of shallow carbonate platforms. The Yucatan Peninsula and the Bahamas are perfectly set up to accommodate the formation of blue holes. The Great Blue Hole is located in Belize, at the Lighthouse Reef Atoll. It is about 124 meters deep and is an interesting part of history. The ancient walls and fossils make this a must see for seasoned divers. One of the inland karst formations is located in the United States. The state of Ohio houses one of these variations. Dean’s Blue Hole is another tourist attraction that is located in the Bahamas. The Infamous Blue Hole in Dahab, Egypt is home to a well-known tunnel called “The Arch.” This tunnel sits at a depth of 184 feet and is about 78 feet long. It is meant for well-trained divers, or those with at guide.

Blue Holes are a remnant of our planet’s history. They can give us some ideas of how the Earth supported life during the ice ages. They can also give clues to future changes influenced by erosion and climate variations. Divers often flock to blue holes around the world to catch a glimpse of history and expand their diving experience. Blue holes may not support life, but they attract researchers and tourists at high rates.

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