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<\/p>\nIn the South China Sea, near the Paracel Islands, the aqua-colored waters of an expansive shallow reef platform suddenly gives way to a near vertical shaft of vast darkness \u2013 an ocean sinkhole almost entirely devoid of oxygen and, in turn, marine life as we know it.<\/p>\n
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The edge of the blue hole can be seen from a boat <\/p>\n
Huanqiu<\/p>\n
This is the Dragon Hole, or more formally Sansha’s Yongle Blue Hole (YBH), which stretches 162.3 m (535 ft) at its widest point and plunges 301 m (998 ft). And there’s still very little we know about life inside the hole, despite the abundant reef biodiversity that surrounds it. There’s no algae, plant life or fish that can survive deeper than about 100 m (330 ft) from the surface. <\/p>\n
In 2016, Chinese scientists became the first to measure the blue void, however, it’s been around for centuries. One of China’s most popular novels, Journey to the West, features it as where the Monkey King Sun Wukong found his golden cudgel. Considering this 16th-century tale, attributed to Wu Cheng’en, has details of it, it’s safe to say the Dragon Hole has been part of folklore for a long time. As far as we can determine, scientists believe the carbonate karst hole was shaped when sea levels were lower, with rainwater dissolving limestone and carving vertical “steps” into the hole’s “walls.” When the sea level rose again, the chasm flooded, creating the submerged sinkhole (blue hole). <\/p>\n
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The Dragon Hole’s external shapeLi, T, Feng, A, Liu, Y, et al\/Nature\/(CC By 4.0<\/a>)<\/p>\n At the time of its 2016 mapping, it was deemed the deepest of its kind on the planet, eclipsing Dean\u2019s Blue Hole in the Bahamas, Egypt’s Dahab Blue Hole, the Belize Great Blue Hole and Malta’s Gozo Blue Hole. While preliminary studies now suggest that Taam Ja\u2019 off the coast of Mexico\u2019s Yucat\u00e1n Peninsula<\/a> has taken the record as deepest ocean sinkhole \u2013 at an estimated 420-m (1,378-ft) depth \u2013 we don’t yet know a lot about life inside that one. <\/p>\n And there are a few features that make Dragon Hole unique. Unlike most underwater caves or sinkholes tied to land-based systems, this blue hole is found in an offshore coral reef platform, far from land. And it’s not its depth that makes it extraordinary \u2013 it’s what is inside it. Below that surface layer that houses some adaptable marine life, there’s not much else, because the water remains stagnant and devoid of oxygen and light.<\/p>\n In most parts of the ocean around the world, wind, waves and currents mix surface waters downward, delivering oxygen to deeper areas. But Dragon Hole’s steep walls and narrow opening prevent such mixing, so the water inside has stratified into distinct layers that rarely, if ever, interact with the surrounding sea water. <\/p>\n Measurements by Chinese marine scientists<\/a> from institutions including the First Institute of Oceanography (FIO) and local Sansha teams conducted the initial comprehensive surveys of Yongle Blue Hole. The study revealed that oxygen concentrations drop rapidly with depth, reaching zero well above the midpoint of the hole. Below this threshold, the water is permanently anoxic \u2013 incapable of supporting fish, plants or algae. It does, however, show signs of diverse microscopic life able to forego sunlight for energy. <\/p>\n Hole wall 3D topography illustrating staircase of steep faces Li, T, Feng, A, Liu, Y, et al\/Nature\/(CC By 4.0<\/a>)<\/p>\n Researchers have identified microbial communities living in the anoxic depths, sustained not by the Sun but chemical energy. These microbes rely on chemosynthesis \u2013 a process in which organisms derive energy from chemical reactions involving compounds such as sulfur, rather than through photosynthesis \u2013 not unlike the microorganisms found in the spider-filled Sulfur Cave<\/a>.<\/p>\n Such marine ecosystems are rare and poorly understood, and Dragon Hole offers scientists a natural laboratory for studying biological extremes. Similar microbial processes are thought to resemble some of the earliest life systems on Earth \u2013 and possibly those that could exist on other planets with subsurface oceans<\/a>.<\/p>\n In 2023, scientists expanded our knowledge of low-oxygen marine environments<\/a> and microbial survival by studying the Dragon Hole where they discovered a fascinating mix of species defined by distinct vertical layers. In the hole, life drops away quickly as oxygen disappears, below the surface (Oxic Zone) and transition levels (Suboxic Zone); at 100-140 m (330-460 ft), the third layer called the Anoxic Zone I is almost completely dominated by sulfur-oxidizing bacteria. At the deepest part of this layer, these bacteria make up around 90% of all microbes in the water. Instead of relying on oxygen or sunlight, these organisms use sulfur compounds as an energy source, through a process known as chemosynthesis. The dominant groups were found to be colorless sulfur bacteria and purple non-sulfur bacteria, with two genera in particular standing out \u2013 Thiomicrorhabdus and Sulfurimonas. These microbes are specially adapted to oxygen-free, chemically harsh environments \u2013 the kind that’s otherwise deadly for marine life.<\/p>\n Below 140 m (460 ft), the environment shifts into Anoxic Zone II, a deeper and chemically more stable layer that extends to the bottom. Here, nitrate disappears entirely and hydrogen sulfide accumulates, signaling a fundamental change in how energy flows through the system. Sulfur oxidation gives way to sulfate reduction, and microbes becomes both more diverse and more strictly anaerobic. Desulfatiglans emerges as a dominant player, alongside sulfate-reducing bacteria such as Desulfobacter, Desulfovibrio, and Desulfobulbus, organisms that actively generate hydrogen sulfide as part of their metabolism.<\/p>\n This zone also hosts a broader mix of anaerobic specialists, including green sulfur bacteria such as Prosthecochloris, as well as groups like Chloroflexi and Parcubacteria. While life here is metabolically slower than in the zone above, it’s more diverse and forms a tightly coupled, self-contained sulfur cycle with no connection to the oxygenated ocean near the surface.<\/p>\n Together, these stacked anoxic layers turn the Dragon Hole into something more than a vertical cave in the sea, with stratified microbial ecosystems, each governed by their own chemistry and energy requirements for survival.<\/p>\n Determination of the depth of the blue holeLi, T, Feng, A, Liu, Y, et al\/Nature\/(CC By 4.0<\/a>)<\/p>\n To understand this ecosystem more deeply, the researchers extracted samples and attempted to grow microbes back in the lab. They successfully isolated 294 different bacterial strains using a range of growth conditions. Notably, among the anaerobic (oxygen-free) bacteria, 22.2% appeared to be previously unknown species, suggesting Dragon Hole may host entirely new forms of life. The study also found a clear difference between free-living bacteria, which float independently in the water column, and particle-attached bacteria, which cling to sinking debris. This split suggests the microbes have multiple survival strategies that work alongside each other in this extreme environment.<\/p>\n![\"Hole](\"https:\/\/www.newsbeep.com\/nz\/wp-content\/uploads\/2026\/01\/1767474869_63_.webp\")
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