Dragon Hole, also known as Yongle Blue Hole named after the third

Ming The Ming dynasty (), officially the Great Ming, was an imperial dynasty of China, ruling from 1368 to 1644 following the collapse of the Mongol-led Yuan dynasty. The Ming dynasty was the last orthodox dynasty of China ruled by the Han peop ...

emperor, is the deepest known

blue hole A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock ( limestone or coral reef). Their existence was discovered in the late 20th century by fishe ...

in the world at deep. It is located about north of Drummond Island in the

Paracel Islands The Paracel Islands, also known as the Xisha Islands () and the Hoang Sa Archipelago ( vi, Quần đảo Hoàng Sa, lit=Yellow Sand Archipelago), are a disputed archipelago in the South China Sea. The archipelago includes about 130 small coral ...

. Blue holes generate a distinctive blue colour when seen from above. They are characteristically only dozens of metres deep. The local fishermen call it the "eye" of the

South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Phi ...

, and according to legend it is where the

Monkey King The Monkey King, also known as Sun Wukong ( zh, t=孫悟空, s=孙悟空, first=t) in Mandarin Chinese, is a legendary mythical figure best known as one of the main characters in the 16th-century Chinese novel '' Journey to the West'' ( zh, ...

, depicted in the novel ''

Journey to the West ''Journey to the West'' () is a Chinese novel published in the 16th century during the Ming dynasty and attributed to Wu Cheng'en. It is regarded as one of the greatest Classic Chinese Novels, and has been described as arguably the most popul ...

'', found his golden cudgel. Dragon Hole is about deeper than

Dean's Blue Hole __NOTOC__ Dean's Blue Hole is a blue hole located in The Bahamas in a bay west of Clarence Town on Long Island and is the world's second deepest, after the Dragon Hole in the South China Sea, with a depth of . Formation A blue hole is a wat ...

in the

Bahamas The Bahamas (), officially the Commonwealth of The Bahamas, is an island country within the Lucayan Archipelago of the West Indies in the North Atlantic. It takes up 97% of the Lucayan Archipelago's land area and is home to 88% of the ar ...

. There are several fresh water sinkholes on land that are deeper than Dragon Hole. These include Mexico's

Zacatón Zacatón is a cenote (a thermal water-filled sinkhole) belonging to the Zacatón system - a group of unusual karst features located in Aldama Municipality near the Sierra de Tamaulipas in the northeastern state of Tamaulipas, Mexico. It is the d ...

(), Pozzo del Merro in Italy () and Hranice abyss in the Czech Republic (). The Sansha Yongle Blue hole was created around the same time as the surrounding

islands. Blue holes can be formed in a number of ways. The most common is melting ice structures in the surface resulting in large pits where ice once was. Blue holes may also be formed after a disruption in the surface of the ocean floor by tectonic shifts that result in the settlement of sand and debris (Emery, 1962). The Blue Hole in Belize was once a dry land cave; its present form is a result of

sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...

and subsequent roof collapse.

Discovery

The exact location of the Dragon Hole is N16°31.55’. E111°46.1’. Geographically the hole is found in the eastern Atoll near the

, in the South China Sea. According to the Xinhua News agency, upon discovery by local fishermen, the first research methods included utilizing a Video Ray Pro 4 underwater robot. This robot found the depth of the hole to be and a width in some places of . Over 20 new species of sea life were discovered during the initial exploration efforts. Other information that was gained from initial exploration was the distinct difference in chemical makeup between water columns. At the 300 ft mark the levels of oxygen fall off making it more difficult for life to survive. This information leads to speculation on what can be learned from researching the Dragon Hole.

Research

Due to the surrounding geological challenges, new research methods were created in order to study the hole. The surrounding coral is only at an average depth of 0.5-0.8 m, therefore discouraging any large research vessels from accessing the opening of the Dragon Hole. One new method was the utilization of a boat that had a flat bottom that allowed it to rest above the entrance to the hole despite the shallow waters surrounding it. This vessel is used as a staging platform for gathering data, as well as deploying the

Remotely Operated Vehicle A remotely operated underwater vehicle (technically ROUV or just ROV) is a tethered underwater mobile device, commonly called ''underwater robot''. Definition This meaning is different from remote control vehicles operating on land or in the a ...

(ROV). Due to the utilization of a “underwater integrated navigation system” the ROV was capable of receiving real-time commands allowing for control over movement and observation. The device carrier or flat bottom vessel served as an important tool to transport sampling and research equipment on site without damage of the surrounding reefs and environment. The total size of this research vessel was 25x18 m. The ROV or device was a total size of 3.0x1.7x1.7 m. What was specifically used was a “FCV2000D professional grade ROV”. Blue holes are very valuable to scientists regarding the research of climate change, marine ecology as well as geochemistry and diversification between water columns. Throughout the research conducted on the Dragon Hole, a multitude of data was collected.

Tidal measurements

Tide gauge stations were temporarily deployed around the Dragon Hole as well as on the research vessel. This measurement technique resulted in an accuracy within 3 cm.

Surveying

The utilization of the ROV allowed for three-dimensional topography of the entire blue hole. The ROV, equipped with a multibeam device, made scans of the entirety of the underwater system. Combining this with digital scans of the surface of the hole, a three-dimensional model has been created.

Hydrochemical properties

The Sansha Yongle Blue hole, being the deepest of its kind, allows for the largest data collection surface available for this specific research. Through gathered data, it became evident to researchers that two

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

s were distinguishable within the water column. The first transition between warmer mixed water and colder stagnant water occurs between 13 and 20 m. The second thermocline occurs lower in the water column at around 70–150 m. Sampling of water from different depths and layers allows for the gathering of information about rock sediment, minerals, bacteria, organisms, and chemical research. The study of this information can be used to discover critical data about the history of the world dating back to when the Dragon Hole was formed

References

{{Blue holes Sinkholes of Asia Paracel Islands South China Sea Sea caves