Dragon Hole

Dragon Hole is the deepest known blue hole in the world at 300.89 metres (987.2 ft) deep. It is located about 25 kilometres (16 mi) south of Discovery Reef in the Paracel Islands.[1] Blue holes generate a distinctive blue colour when seen from above. They are characteristically only dozens of metres deep.

Dragon Hole
LocationParacel Islands
CoordinatesN16.5249373, E111.7680205 (N16°31.55’. E111°46.1’)
Depth300.89 metres

The local fishermen call it the "eye" of the South China Sea, and believe it is where the Monkey King, depicted in the novel Journey to the West, found his golden cudgel.[1][2]

Dragon Hole is almost 100 metres (330 ft) deeper than Dean's Blue Hole in the Bahamas. There are several fresh water sinkholes on land that are deeper than Dragon Hole. These include Mexico's Zacatón (335 metres (1,099 ft)), Pozzo del Merro in Italy (392 metres (1,286 ft))[3][4] and Hranice abyss in the Czech Republic (404 metres (1,325 ft)).

The Sansha Yongle Blue hole was created around the same time that the surrounding South China Sea islands were created [5]. 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. Some cases of blue holes are also 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).

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 Paracel Islands, in the South Chinese Sea. [5]

According to the Xinhua News agency, upon discovery by local fisherman, first research methods including utilizing a Video Ray Pro 4 underwater robot. This robot found the depth of the hole to be 987ft, and in some places 426ft wide. Over 20 new species of sea life were discovered during the initial exploration efforts[6]

Other information that was gained from initial exploration was the distinct difference in chemical makeup between water columns. At the 300ft mark the levels of oxygen fall off making life more difficult to survive. This information leads to speculation on what can be learned from researching the Dragon Hole[6]

Research

Due to the surrounding geological challenges, new research methods were created in order to study the hole. The surrounding coral only yields on average 0.5-0.8m of depth therefore discouraging any large research vessels from accessing the opening of the Dragon Hole.[5]

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 utilized as staging platform for gathering data, as well as deploying the Remote Observation Vehicle (ROV).  Due to the utilization of a “underwater integrated navigation system” the ROV was capable to receive real-time commands allowing for control over movement and observation[5].  

The device carrier or flat bottom vessel served as an important tool to transport sampling and research equipment Onsight without damage of the surrounding reefs and environment[5]. The total size of this research vessel was 25x 18 metres.  

The ROV or device was a total size of 3.0x1.7x1.7 metres. What was specifically used was a “FCV2000D professional grade ROV” [5]

Research Results

Blue holes are very valuable to scientists regarding the research of climate change, marine ecology as well as geochemistry and diversification between water columns[7]. 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 digital scans of the surface of the hole a three-dimensional model has been created[5]

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 thermoclines were distinguishable within the water column. This first transition between warmer mixed water and colder stagnant water occurs between 13-20 metres. The second thermocline occurs lower in the water column at around 70-150 metres[7]. Sampling at water from different depths and layers allows for the gathering of information about rock sediment, minerals, bacteria, organisms, and chemical research [7]. 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[7]

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References
  1. "Researchers just discovered the world's deepest underwater sinkhole in the South China Sea". The Washington Post. 26 July 2016. Retrieved 26 July 2016.
  2. "China Exclusive: South China Sea "blue hole" declared world's deepest". New China. Xinhua News Agency. 23 July 2016. Retrieved 29 July 2016.
  3. Mazza, Ed (29 July 2016). "'Dragon Hole', World's Deepest Blue Hole, Discovered In South China Sea". Huffington Post. Retrieved 29 July 2016.
  4. "Deep Blue Hole Found in South China Sea". The Wall Street Journal. Dow Jones. 28 July 2016. Retrieved 29 July 2016.
  5. Li, Tiegang; Feng, Aiping; Liu, Yanxiong; Li, Zhenhong; Guo, Kai; Jiang, Wenzheng; Du, Jun; Tian, Ziwen; Xu, Wenxue; Liu, Yang; Wang, Yanru (2018-11-20). "Three-dimensional (3D) morphology of Sansha Yongle Blue Hole in the South China Sea revealed by underwater remotely operated vehicle". Scientific Reports. 8 (1): 17122. Bibcode:2018NatSR...817122L. doi:10.1038/s41598-018-35220-x. ISSN 2045-2322. PMC 6244355. PMID 30459420.
  6. "Researchers just discovered the world's deepest underwater sinkhole in the South China Sea". The Independent. 2016-07-27. Retrieved 2020-04-22.
  7. Xie, Linping; Wang, Baodong; Pu, Xinming; Xin, Ming; He, Peiqing; Li, Chengxuan; Wei, Qinsheng; Zhang, Xuelei; Li, Tiegang (2019-02-01). "Hydrochemical properties and chemocline of the Sansha Yongle Blue Hole in the South China Sea". Science of the Total Environment. 649: 1281–1292. Bibcode:2019ScTEn.649.1281X. doi:10.1016/j.scitotenv.2018.08.333. ISSN 0048-9697. PMID 30308898.
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