China has identified a rare coral reef blue hole within the Scarborough Shoal, a hotly contested feature in the South China Sea that is now a major flashpoint in the region.

A marine blue hole is a deep, circular sinkhole in the ocean and is typically bright turquoise or emerald green.

Unlike most of the world’s known blue holes – which were once caves on land thousands of years ago during the last Ice Age – the newly discovered marine blue hole within the Scarborough Shoal lagoon was formed by the natural growth of a coral reef, a report published by the Ministry of Ecology and Environment said on Thursday.

Chinese researchers identified a rich diversity of marine life in and around the blue hole, which dated back at least 3,200 years, the report said. The discovery is expected to offer new insights into the geological evolution and environmental history of the shoal.

The findings were released at a time when Beijing is pushing to further solidify its control over the contested waters, leveraging a mix of law enforcement patrols and scientific expeditions.

Sitting about 230 km (143 miles) off the Philippine coast and about 874 km from southern China’s Hainan province, the Scarborough Shoal is a triangular-shaped reef formation around a lagoon. It is known as Huangyan Island in China and Panatag Shoal in the Philippines, which also claims the feature.