Blue holes, or offshore flooded caves are aptly named for the dramatic contrast between the dark blue waters of their depths and the lighter blue of the surrounding shallower waters. They are underwater sinkholes that were formed during glaciations when sea levels were much lower than today. As sea levels rose, the caves were flooded, leading to the creation of these ocean abysses.

Such dramatic marine landscapes can be hosts to a wealth of marine life and create a dive that is truly unforgettable. I certainly haven't forgotten ascending from the stalactites at 40m in the Blue Hole in Belize, doing a safety stop amongst circling grey reef sharks. Other famous offshore blue holes include those in Dahab, Egypt; San Lawrenz, Gozo and the Bahamas.

And then there are the inland blue holes such as Stargate, found on Abaco Island in the Bahamas. These are a very different kettle of fish both in terms of what they have to offer and how difficult they are to dive. Actually understanding how they are formed is a science in itself! A layer of freshwater (from rainfall) sits on top of the denser seawater below, which in turn prevents any immediate contact with oxygen. This creates a layer in which only bacteria can thrive, and these bacteria produce toxic hydrogen sulphide as a waste product.

Diving through this toxic layer is not for the faint hearted, or indeed the untrained. Tales of skin itching, tingling or dizziness are common. Some divers report the smell of rotten eggs as the gas penetrates their skin pores and metabolises through the lungs. It makes sense then that diving something like Stargate requires as much preparation as preparing to climb Everest and should only be undertaken by professionals.

It's probably a fair question to ask why do these divers risk so much to dive these hostile environments? They are not just adrenaline junkies. Funded by the National Geographic, a group of scientists embarked on the Bahamas Blue Hole Expedition where they tallied up about 150 dives over 7 different Bahamian islands. The rationale behind this expedition was to uncover many of the secrets that these depths hold on our limited understanding of geology, water chemistry, biology and even astrobiology (the study of life in the universe). As these blue holes are able to preserve bones and skeletons owing to their anoxic (oxygen absent), sheltered conditions, it's easy to see why they act like magnets for scientists all over the world. Its just a shame that not even these environments are untouched by our human footprint. With rising sea levels linked to global warming, this expedition could be the last of its kind as inland blue holes are threatened by the risk of flooding.

Contact us to find out how you can dive an offshore blue hole (it seems best to leave the inland blue holes to the experts!)