The discovery of a “blue hole” off Abu Dhabi has linked the UAE to nations across the globe that are also home to stunning marine sinkholes.
From Belize to the Bahamas, from China to Malta, blue holes are breathtaking sights often characterised by water that, because of the hole’s depth, appears a much darker blue than the surrounding sea.
Typically formed in areas of limestone or coral, they can be rich in life nearer the water surface and often contain numerous species of fish.
Some, even if the surface area is smaller, are much deeper than Abu Dhabi's newly revealed blue hole, stretching hundreds of metres vertically.
However, with poor circulation, deeper blue holes may be oxygen-depleted further down, where the waters are home to many types of micro-organisms.
Dr Jamie Pringle, a senior lecturer in geosciences at Keele University in the UK, said the mechanism by which blue holes are formed was unusual.
Sinkholes on land are created typically, he said, by the flow of rain or groundwater, which gradually wears material down over time before the areas of erosion connect to form caverns.
The blue holes, by contrast, are thought to be formed by the exposure of structures on land at a time of lower sea levels. The eroded areas are filled by water when sea levels rise.
“It is interesting and unusual as it needs both soluble bedrock that is exposed to the surface for a relatively long time, then is covered with water to preserve [the blue hole],” Dr Pringle said.
The Abu Dhabi blue hole, which lies off Al Dhafra, is skirted by coral and is about 200 metres wide and with a depth of about 12 metres it is relatively shallow compared to other large blue holes. Like others, it is home to many fish species, including in this case emperor fish and jackfish.
No less stunning than blue holes are some of the sinkholes that form on land, such as cenotes, a name usually used in Mexico for pools formed when limestone bedrock – the solid rock that sits underneath the soil – collapses.
These are filled with groundwater and in come cases become popular swimming spots. The term 'cenote' is sometimes applied to blue holes, too.
Related structures include karst lakes, which are created by the collapse of caves in limestone regions when the underlying bedrock has dissolved.
Here are some of the world’s most impressive blue holes:
The world’s deepest known blue hole is in a coral reef off the Paracel Islands, a disputed archipelago in the South China Sea.
Known as Dragon Hole and the Sansha Yongle Blue Hole, it was described in a 2018 article in the journal Scientific Reports as resembling a ballet dancer’s shoe and has a depth of 301 metres.
Its upper section has a roughly vertical cylindrical shape but further down the hole narrows and is angled diagonally.
Bahama Blue Hole
With a depth of about 202 metres, the Bahama Blue Hole or Dean’s Blue Hole was the deepest known in the world until the Dragon Hole was analysed.
Dean’s Blue Hole has been the location for numerous world records in free-diving, in which participants dive without breathing apparatus.
One record set there, in 2018, was by Alexey Molchanov, a Russian, who went 130 metres below the surface, described by Guinness World Records as “the deepest self-propelled dive in history”.
Blue Hole (Sinai Peninsula)
Known simply as Blue Hole, a site at Dahab off the Sinai Peninsula on Egypt’s Red Sea coast has a depth of slightly more than 100 metres and is a popular diving location.
There have been numerous deaths at Blue Hole, partly because it contains a tunnel known as the Arch, which is said to be hazardous.
It is also a popular spot for snorkelling and tourism companies offer trips, with visitors able to enjoy a variety of multicoloured coral and many fish species.
Great Blue Hole
The Great Blue Hole off Belize in Central America lies near the centre of a coral atoll and was made famous by Jacques Cousteau, the French ocean explorer.
Famously, Cousteau berthed his ship, the Calypso, in the blue hole in the early 1970s and detected stalactites beneath the surface, indicating that the structure had formed on land before a rise in the sea level.
Another popular diving spot, it lies about 40 miles from the coast of Belize and is part of a reef system that is a Unesco World Heritage Site.
Blue Hole (Gozo)
Another site called Blue Hole is found off the Maltese island of Gozo and is a popular location for diving, free diving and snorkelling.
It is a sinkhole in the limestone that measures about 10 metres by five metres and is sheltered from the sea by a fringe of rock.
The Blue Hole has a depth of about 25 metres and at nine metres is an archway that connects to the open sea.