“Blue holes” are a haven for both deep divers and researchers, though for different reasons. For a deep diver, it would mean an adventure and an experience like no other; for a scientist, its an insight into the past as well as the future. Blue holes are very deep and with no oxygen or light, and they possess one of the harshest environments that could exist on Earth. Two years ago, Chinese researchers discovered the deepest of blue holes ever found so far—Dragon Hole. Apart from a topographic survey, it is as yet unexplored and unventured.

Located about 25 kilometers from the Discovery Reef in the South China Sea, the Dragon Hole is the deepest known underwater sinkhole in the world. It is believed to be 301.19 meters deep, beating Dean’s Blue Hole in the Bahamas by almost 100 meters.

The Discovery Reef is part of the Paracel Islands, also known as “Xisha” in Chinese and “Hoàng Sa” in Vietnamese, which is a group of almost 130 islands, reefs, and banks in the South China Sea scattered across 15,000 square kilometers (5,800 square miles). It is also home to Dragon Hole, now officially known as the “Sansha Yongle Blue Hole (SYBH).”

Researchers have examined the blue hole in 2016 and obtained an estimated depth of approximately 300.89 meters relative to the sea’s surface. A further survey was conducted in 2017 using a professional, underwater, remotely operated vehicle (ROV) revealed the depth to be 301.19 meters below the 10-year average of sea level.

The 2017 survey revealed that SYBH is shaped like a ballet dancer’s shoe, and its entrance is shaped like a comma with the widest part being 162.3 meters and the narrowest being 26.2 meters. The blue hole has a volume of almost 500 cubic meters. 

From May 15 to June 5, the researchers from First Institute of Oceanography, State Oceanic Administration, China, conducted a thorough survey to determine the three-dimensional morphology of the blue hole. They obtained the precise depth of the hole, complete shape data for the cave walls, and also found that it’s not a true vertical hole, but instead inclines at places and forms five distinct sections.

The reef on which the Dragon Hole is located is only submerged in waters during the spring high tide, and at low tide, it is above sea level. The high tide lasts only for approximately two hours, and even then, the water depth is just one meter. This posed a challenge for the research team in bringing their boat and equipment onto the blue hole’s entrance as the reef is fragile. So, they had to construct a flat-bottomed boat that could carry their devices.

Blue holes formed when the sea levels on Earth were as low as 120 meters during previous ice ages due to the same weathering and erosion that causes sinkholes on land. When the ice melted, they became flooded with water. 

The blue holes hold great value to the scientists as they give information about Earth’s geological past. During the ice ages, ice sheets in Europe, North America, and Siberia were as thick as four kilometers. What water remained eroded the limestone-rich land, something known as the “karst process,” creating large cavities whose ceiling would fall down due to its own weight creating sinkholes.

Unlike sinkholes, blue holes are not completely vertical. Since blue holes contain both freshwater and saltwater, a corrosive reaction occurs where the two waters meet. This creates horizontal caves or “arms” that extend from the vertical cave. These arms can be quite long, one of the longest being the South Passage of Sawmill Sink in the Bahamas which is 762 meters.

Dragon Hole does not have any open, horizontal caves that could let the ocean tides in resulting in isolated and very stagnant deep waters. Because of this, the water below 80 meters is devoid of oxygen. 

The tidal currents and surface waves can only reach up to a depth of 60 meters below the sea level of an enclosed cave. According to the Chinese research team, not only are the walls of Dragon Hole closed, but there is also sediment and silt at the cave’s bottom suggesting minimal to no exchange of water. The only time ocean water comes into contact with its water is during high tide. In the case of landlocked blue holes, the freshwater from rain pools on top of the saltwater forming a barrier that blocks atmospheric oxygen preventing it from mixing with deep waters.

The oxygen-free conditions of blue holes, however, are not an impediment to all lifeforms. The Sawmill Sink, for example, is home to thriving anaerobic bacterial colonies as well as quite a number of fossils perfectly preserved by the anoxic waters.

According to Jenn Macalady, an astrobiologist from Pennsylvania State University, understanding the bacteria found in Sawmill Sink can help postulate the existence of lifeforms in faraway planets and moons that do not have Earth-like conditions. The bacteria and anaerobic decay of organic matter release hydrogen sulfide which could be mildly toxic to the divers.

So far, divers found fossils of various animals including tortoises, crocodiles, bats, owls, and beetles from thousands of years ago in Sawmill Sink. Human skeletons, possibly of Lucayan people, were also found. Since Dragon Hole was only recently discovered, its bottom might also prove to hold clues to what life was like thousands of years ago.