How Blue Holes Are Formed

blue hole bay, long island, bahamas

Blue Hole Bay – Long Island, Bahamas

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.

andros island bahamas blue hole

Blue Hole – Andros Island, Bahamas

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, Marie-Ange Ostre



  1. Nope. Where’s this one? What are the details on it?

  2. Does Blue Grotto in Florida count? :)

  3. Just did Belize last summer!!!

  4. So pretty!!!

  5. Bahamas!!

  6. Going to Guam next week, and will dive one there.

  7. Yes I have, the blue holes in the Bahamas are awesome! Try Dan’s Cave or The Magic Blue Hole (Cave training required)

  8. It’s deans blue hole. There’s a Vertical Blue free diving comp going on there right now.

  9. Did Blue Hole Dahab last Spring……

  10. Belize!

  11. Yeah, she’s not sad anymore :)

  12. Did the Blue Hole Belize eight years ago and passed the dive last year aboard the Sundancer once was enough

  13. Packing for Belize. We leave in the morning.


  15. Guam

  16. Daniel have fun, I’m on the Sundancer again in June can’t wait

  17. Guam and Palau, I miss diving!

  18. In 5 weeks!

  19. Palau and Dahab- fantastic!

  20. Guam and the best time to do it on a full moon night dive with no lights it’s incredible

  21. Dove a lot of pink ones. Even a brown one once or twice.

  22. Yes

  23. Dove the blue hole in belize. Best dive ever!

  24. Yes!!

  25. beautiful

  26. No but soon

  27. No but soon

  28. No. But did dive a cenote

  29. No. But did dive a cenote

  30. Belize

  31. Belize

  32. IN DAHAB

  33. IN DAHAB

  34. This is the Private Blue Hole in the Bahamas, where free divers do their thing.

  35. Bahamas has a ton of blue holes.. My favorite location is Cay Sal Banks…

  36. Not like this one!

  37. Belize -crossed it off the bucket list!

  38. Malta

  39. I went to a lecture by Ken Broad about his research and exploration of blue holes. Fascinating time capsules of ancient oceans.

  40. On a regular basis Blue Hole Santa Rosa New Mexico.

  41. Yup…145′ in Belize.

  42. No…but willing to have someone sponsor me so I can go.

  43. Belize. THE blue hole!

  44. Yep!

  45. Belize Blue Hole is great, but a fast down, and a short dive. Amazing though. You mention the cenotes in Mexico. Some have the halocline where salt water & fresh water meet. Amazing to dive through. Yey.

  46. Belize

  47. Planning on it next summer!

  48. Been to Belize twice…no Blue hole either time. Will keep trying

  49. no I have not

  50. and I do not wont to

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