Today in Science and Technology History | The world's deepest known ocean blue hole is in Sansha, my country

Ocean blue holes are rare natural geographical phenomena on Earth. From the sea surface, blue holes present a different deep blue color from the surrounding waters, and form huge deep holes on the seabed. Scientists call them "the last legacy of the Earth to mankind to preserve the secrets of the universe". On July 24, 2016, the world's deepest ocean blue hole was discovered in Sansha. After on-site investigation and confirmation by experts, the Sansha Municipal Government officially named the ocean blue hole in Yongle Atoll of the Xisha Islands as "Sansha Yongle Dragon Cave" on the afternoon of the 24th. It was confirmed to be the deepest known ocean blue hole in the world, located in the reef of Jinqing Island and Shiyu Island of Yongle Atoll of the Xisha Islands in Sansha City, with a depth of 300.89 meters.

●Refresh the record of ocean blue hole depth

What is a blue hole? According to Dr. Zhang Yuyang from the South China Sea Institute of Oceanology, Chinese Academy of Sciences, a blue hole is a rare natural geographical phenomenon. It is a huge abyss where the seabed suddenly sinks. Compared with the surrounding waters, it presents a dark and mysterious deep blue tone.

Blue holes are rare natural geographical phenomena on Earth. According to different environments, people divide them into ocean blue holes and land blue holes. From the sea surface, ocean blue holes appear dark blue, which is different from the surrounding waters, and form huge deep holes on the seabed. Scientists call them "the last legacy of the Earth to keep the secrets of the universe for mankind."

The depth of the world's previously discovered ocean blue holes is ranked as follows: Deans Blue Hole on Long Island, Bahamas (202 meters), Hadab Blue Hole in Egypt (130 meters), Great Blue Hole in Belize, Honduras (123 meters), and Gozo Blue Hole in Malta (60 meters). The depth of the Yongle Blue Hole in the Xisha Islands has set a new record for the world's ocean blue holes.

Sansha Yongle Dragon Cave is located in the reef of Jinqing Island and Shiyu Island in Yongle Atoll of Xisha Islands, Sansha City. The address coordinates are 16°31′30″N, 111°46′05″E. Looking down from the air, the entire Dragon Cave is like a blue-green gem inlaid on the blue waters of the South my country Sea.

"Sansha Yongle Dragon Cave" has a long legend. Fishermen in Tanmen, Hainan, have been fishing in the Xisha Islands for generations. They heard from their predecessors that there is an unfathomable "Dragon Cave" in Yongle Atoll of the Xisha Islands. There are many myths and legends about the Dragon Cave among Hainan fishermen. Some fishermen say that Yongle Dragon Cave is the eye of the South China Sea and contains the treasure of the South China Sea. Some say that this is the location of the Dinghai Shenzhen. Sun Wukong pulled out the Dinghai Shenzhen to make the Ruyi Jingu Bang, leaving behind the unfathomable Dragon Cave. Fishermen have respected and cherished the Dragon Cave for generations.

●New discoveries in Yongle Dragon Cave

What secrets does the mysterious Yongle Dragon Cave hide?

In order to unveil its mystery, with the support of the Sansha Municipal Government, the Sansha Track Coral Reef Protection Institute went to the Xisha Islands four times between August 2015 and June 2016, using sonar side-scan equipment, electronic counting plumb bobs, deep-sea current meters, deep-sea water quality analyzers, underwater robots, underwater photography and video equipment and other equipment, and combining scientific equipment detection with manual diving observation and measurement, to conduct scientific exploration of the Yongle Dragon Cave, obtain a large amount of original scientific data, and successfully explore the depth and basic shape of the Yongle Dragon Cave.

In addition to the depth of the Longdong, researchers also collected biological specimens in the Longdong and conducted preliminary explorations on the composition of the Longdong's cave walls, basic forms, and the organisms contained therein. The results showed that the Longdong's cave entrance was in the shape of a bowl, and was basically a vertical cave, with an obvious northward turn and inclination below a depth of 160 meters. At different depths, the cave walls gradually narrowed or tilted. Researchers found more than 20 species of marine fish and other common marine animals in the cave, including rough sand stingrays, lionfish, parrotfish, flat-backed pufferfish, giant clams, etc., which were the same as the species in the surrounding waters. Preliminary exploration also showed that so far, no side caves connected to the open sea in the Longdong have been directly observed by the naked eye or using sonar, video and other equipment. Starting from a depth of more than 110 meters, the dissolved oxygen content in the water of the Longdong is almost zero, and experts call it "a rare highly compressed natural marine laboratory in the world." Human activities will seriously affect the extreme oxygen-free environment in the Longdong cave, and once destroyed, it will be difficult to recover.

On July 5, 2016, at the "Xisha Blue Hole Exploration" project discussion and review meeting, the expert group headed by Meng Wei, an academician of the Chinese Academy of Engineering, unanimously agreed that the Xisha Blue Hole is a unique marine geomorphic unit with extremely high value. The methods and equipment used in this exploration are advanced and reliable, and the exploration results are credible. They also emphasized that the project is of great significance in safeguarding national sovereignty and marine rights and interests, conducting marine scientific research, spreading marine culture, and supporting marine engineering construction.

On July 24, 2016, Gai Guangsheng, director of the Publicity and Education Center of the State Oceanic Administration and an expert of the Sansha Ecological Environment Protection Committee, dived into the Dragon Cave and found a rope-like, two-meter-long marine creature that had never been seen before at a depth of about five or six meters. "Its head was stuck in a hole in a reef, its body was crooked, and it was covered with armor, which should still be soft," said Gai Guangsheng. During this dive, Gai Guangsheng also found that between the reefs, there were thin lines connected like a pearl net, and the lines were covered with flocs. The entire rock wall looked and felt similar to a hillside on land where a mudslide had just occurred. He believes that the unique ecosystem and natural conditions of Sansha Yongle Dragon Cave are of great significance to the study of marine geological environment. "If we take up its rock layers layer by layer for testing, we can conduct chronological research and restore the ancient environment in the past," said Gai Guangsheng.

●Blue holes are not forbidden areas for life

To this day, Yang Jianhui, a member of the South China Sea Institute of Oceanology, Chinese Academy of Sciences, cannot forget the solitude of the blue hole. It is not easy to approach the blue hole: the reef where Jinqing Island is located is like a mermaid with a long fish tail. At the bend of the fish tail in the northeast corner is the blue hole. The waterway is very shallow and can only be entered at high tide.

"On September 13, 2013, we slowly approached the top of the blue hole in a speedboat. Gradually, the reef flat was like a funnel, from a gentle downhill slope to a cliff-like drop, and the sea water suddenly changed from shallow to deep. We seemed to be slowly 'sliding' into the blue hole." Yang Jianhui said, "Because we didn't know the depth, we didn't dare to dive from the center of the blue hole, and only dived along the wall. Unlike diving in general sea areas, after entering -20 meters, a cliff appeared and the water temperature dropped significantly. When we dived to -30 meters, it was pitch black when we looked into the depths. Fear and coldness surrounded me, and I was left with only awe for nature in my heart, and I didn't dare to dive any further."

The Sansha Track report shows that through visual observation, a variety of fish and other marine life similar to those in the surrounding waters were found in the upper layer of the cave, and there are also corals and other primitive ecological biota in the depths. Therefore, the blue hole is by no means a barren land. Some people have also suspected that there is an independent freshwater area in the blue hole. The Sansha Track report did not respond to this. Zhang Yuyang believes that the freshwater layer must not exist, because freshwater has a small specific gravity and must float on the seawater, and they did not find a freshwater area in the upper layer of the blue hole during the scientific expedition.

How to understand the conclusion of the investigation that "no connection between the blue hole and the open sea has been observed, and there is no obvious flow of water in the cave"? "There is no 'barrier' between the upper layer of the funnel and the open sea, so they are definitely connected. What we are talking about here is whether the side walls and the bottom of the lower layer are connected. Because the walls of the blue hole are made of porous limestone, many people speculate that the sides or bottom of the blue hole are connected to the outside world. Further speculations include water flow in the cave, and mysterious beams of light shining in from different directions." Yang Jianhui said, "But during the dive, I did not feel the water flow or the mysterious beams of light."

Comprehensive from Guangming Daily and Wenhui Daily