An amazing phenomenon on our planet are the Blue Holes, also known as vertical caves or underwater sinkholes. They are named after their intense blue colour, caused by the dramatic contrast of light blue shallows around the edges and intense blue in the middle due to their depth. It is thought that most blue holes are caused by karst processes, which is the chemical dissolution of carbonate rocks. Anyone who is lucky enough to dive or snorkel a blue hole will be wowed by its beauty, water clarity and perfect shape.
The Dean's Blue Hole in the Bahamas is the deepest saltwater blue hole in the world at 202 metres (663 foot) deep and 25-35 metres (50-100 feet) wide. Most blue holes are around 110-120 metres deep making the Dean's Blue Hole quite an exception. It is a very popular free diving location due to its depth and lack of waves or currents and visibility of around 15-30 metres year round. The Dean's Blue Hole is also a popular diving and snorkelling spot with a fabulous cavern at 20 metres that extends the width of the hole to 100 metres (330 feet), with all kinds of reef fish, groupers and snappers in the shallows, a turtle or two and a large school of Tarpon at around 30 metres.
The Bahamas is also home to some amazing fresh water blue holes and some of the best cave diving in the world. The Bluffs area off South Andros is home to the world famous Stargate Blue Hole, an inland hole that only some the most experienced of cave divers have ever explored. There is also the Benjamin's Blue Hole, El Dorado Blue Hole and the Exley's Boiling Hole.
The widest of the blue holes is the Great Blue Hole in Belize, which is a part of the Lighthouse Reef system, measuring 300 metres (984 feet) across and 120 metres (394 feet) deep. Probably the most visited dive location in the Caribbean and one of the top 10 dive sites in the world, the Great Blue Hole is warm, clear, inviting and an awesome place to see sharks. You may encounter various species such as Nurse Sharks, Black tip and Caribbean Reef Sharks and maybe even a Bull Shark or Hammerhead if you're lucky! There are caverns with stalagmites and stalactites and interesting rock formations. Be aware that some guides take you to 40 metres regardless of your certification, so be sure you ask about this with the company before you dive, or do your deep certification before your trip.
Probably the most popular and visited blue hole is in Dahab Egypt. It has been named the most dangerous dive site in the world and has claimed at least 40 lives on government record but local guides believe the number almost double that. Accidents were usually caused by divers trying to find the entrance to 'The Arch' at a depth of 52 metres, which opens to the ocean, but quite difficult to find and due to the depth, people become narked and disoriented, resulting in an accident. Now dives are heavily monitored and no diver is allowed to enter the water without a qualified, CDWS certified guide.
Due to the high number of visitors to Dahab's Blue Hole every year, the corals and sea life are suffering immensely. Most divers start their dive at The Bells and end up in the Blue Hole, which is a lovely dive and the most popular in the area. The brightly coloured corals are alive and healthy and the marine life on this dive is incredible, with every kind of reef fish staring at you inquisitively. Snorkelling from The Bells to the Blue Hole is also very popular but don't be disappointed at the condition of the reef and lack of sea life as you enter the hole.
Malta's Blue Hole is located on the small island of Gozo in the Mediterranean Sea. Although the hole is not clearly visible on the surface, you'll find the entrance at 7 metres which opens into a cave at 15 metres where you can swim through a chimney and arrive at some beautiful coral gardens and loads of sea life. Large schools of tuna can be found around the rocks along with grouper and barracuda lurking in the shadows.
Guam's Blue Hole is one of the area's most popular dive sites. The opening is at 18 metres (60 feet) and the hole descends to 90 metres (300 feet). There is an exit from the shaft at about 40 metres (127 feet), so divers must be aware this is a deep dive and proper training is necessary - always watch your air and depth carefully. Inside the shaft you will find different types of hard and soft corals, morays eels poking their heads out and a variety of fish species. At the exit from the shaft, keep your eyes peeled for large pelagic species and as you ascend you'll encounter schools of fish, triggers and more coral and algae.
There are a couple of known blue holes in Australia but they are quite unexplored and due to the sheer size of the Great Barrier Reef, its certain that more are out there. Wonky Hole is 80 metres (262 feet) deep and an amazing dive teaming with sea life and covered in whip coral and gorgonians. The Barrier Star is another hole dropping to 90 metres (295 feet) with an overhang at the sandy bottom and an abundance of reef life and coral. The weather can be brutal on the Otter Reef so care should be taken in planning these trips.
So how many blue holes will you discover in your diving career? There are so many other fresh water sinkholes that could be classed as blue holes but these mentioned are the most popular. For more information on sinkholes you can check out our cave diving and freshwater diving sections. These naturals wonders are just waiting for you to discover.
(By Kelly Luckman)