Rare coral reef blue hole found in South China Sea

China's first coral reef blue hole — a large natural depression on the ocean floor — has been identified in the waters off Huangyan Island in the South China Sea, and it has emerged as a biodiversity hot spot, according to a report released on Thursday.

Environmental DNA technology has helped scientists detect more than 2,700 marine species in and around the blue hole, which was first spotted by a scientific observation team in August last year in the central area of a lagoon, said the 2025 Survey Report on Huangyan Dao Blue Hole released by the Ministry of Ecology and Environment.

Based on their genesis, these underwater sinkholes can be categorized into two types — one formed through limestone dissolution and the other formed by coral reefs, with the latter considered a rare phenomenon worldwide.

Preliminary geochronological studies suggest that the Huangyan blue hole formed at least 3,200 years ago and developed in a high-density patch reef area within the lagoon, the report said. The sinkhole is shaped like a funnel, with a surface diameter of 56.3 meters and a depth of 16.6 meters. The surface area of the blue hole is 1,491.7 square meters.

Typical coral reef biological communities are thriving inside the blue hole, including different species of fish, giant clams, sponges and sea anemones. The green sea turtle, which is a Class I protected wildlife species in China, has been observed inhabiting the blue hole and its surrounding waters.

Field surveys in the surrounding waters over the past two years have recorded 165 species of hard corals, belonging to 14 families and 44 genera, and 184 species of fish, belonging to 27 families and 85 genera.

The report said this coral reef blue hole serves as a vivid testament to the natural ecosystem diversity of Huangyan Island and a key geological archive for reconstructing paleo environmental changes in the South China Sea, including sea-level fluctuations and the impacts of global and regional climate change.

The blue hole can provide critical insights into the geological evolution of the Huangyan region, as well as biodiversity and environmental changes in the South China Sea since the Holocene epoch, it said.

The sinkhole offers essential scientific support for understanding the formation, evolutionary mechanisms and ecological effects of this unique marine ecosystem, the report added.

Pei Xiaofei, a spokesman for the Ministry of Ecology and Environment, said the results of the survey highlight the progress China has made in its ecological protection efforts in the surrounding waters of the island, which is firmly underpinned by the establishment of the Huangyan Island national nature reserve in September 2025.

The ministry will conduct "more in-depth, systematic and comprehensive research" on the blue hole to provide scientific support for further enhancing ecosystem and biodiversity conservation in the South China Sea, Pei added.