Of all the alluring and mysterious dive sites in the world, blue holes tend to be at the top of many divers’ must-dive lists, and it’s not hard to understand why. From above, these natural formations appear as an eerie portal into the deep unknown, often a deep shade of midnight blue that contrasts sharply with the turquoise waters that typically surround it. They certainly have their place in the folklore of the regions in which they occur, but as it turns out, their origins are not all that mystical. So how is a blue hole formed?
To begin with, blue holes are, quite simply, holes in the earth that are filled with water, which can be a mixture of salt and fresh water. They are named quite literally for the deep blue coloration of the waters within, typically most strongly perceived when viewed from an elevated location. Their color comes from the refraction and reflection of light in the deep column of water they contain. Some of the world’s larger blue holes also feature side passages that lead off of the main chamber into other passageways. Our planet’s deepest such hole is Dean’s Blue Hole in the Bahamas, measured to a depth of over 200 meters.
The formation of these curious features began many millennia ago. During the Ice Age, sea levels were nearly 100 meters lower than they are in present day. Over time, glacial runoff began dissolving the limestone ground, forming sinkholes with massive caverns below. The ceilings of these caverns would eventually collapse, leaving deep holes in their wake. As the glaciers of the Ice Age steadily melted, the deep holes in the ground were filled with water to create the blue holes we are so fascinated with today. Some holes continue to expand from the continued reaction of organic chemicals in the water eating away at the limestone and other rocks around the hole’s column.
Not all of these holes were formed near the ocean. Some were formed in areas that are still landlocked, but contain side passages that lead to the ocean. The passages allow sea water to flow to the hole and fill it with the tides, which is why some inland holes will have saltwater. The inland formation of holes is usually by the same methods as oceanic holes. Some rare holes are far enough inland that they are filled exclusively with fresh water from falling rain or natural runoff, as is the case for many of Mexico’s cenotes.
The life within blue holes varies by geographic location, but the sheer age of the formation in which you can dive is fascinating enough to make blue holes some of the world’s most popular dive destinations.
Have you ever dived a blue hole?
Images via blueholebay.com, Marie-Ange Ostre