Inside the Dragon Hole of the South China sea, where oxygen vanishes from the ocean

The Sansha Yongle Blue Hole, also called the Dragon Hole, reaches 301.19 metres deep in the South China Sea and was once the world’s deepest known blue hole. Detailed 3D mapping in 2017 revealed a complex, bending structure rather than a simple vertical shaft. Below about 90 to 100 metres, oxygen nearly disappears and hydrogen sulphide appears, creating isolated layers that preserve chemical clues about past ocean and climate conditions, making the site a valuable natural archive.

Did our AI summary help?

Scientists continue examining the Sansha Yongle Blue Hole, known as the Dragon Hole, revealing new structural and chemical insights beneath the South China Sea, researchers reported. The site has drawn global scientific attention. It was once considered the deepest blue hole. That depth record has since shifted elsewhere. Yet interest in this formation remains strong.

Dragon Hole Structure Reveals Complex History

The Dragon Hole lies within the South China Sea. It reaches a depth exceeding 301 metres. Earlier surveys confirmed a depth of 301.19 metres. That figure carried only minor uncertainty. The site was once the deepest recorded. A deeper site in Mexico now holds record.

Researchers say the hole bends sideways. It is not a straight shaft downward. The lowest point sits far from entrance. It lies over 100 metres laterally displaced. The upper opening appears wide and circular. Deeper sections narrow and widen unexpectedly.

These irregular shapes suggest gradual formation. Scientists believe sea levels shaped development. It likely expanded during colder periods. Ocean levels once stood much lower. Reef limestone forms most interior walls. Side cavities appear suddenly within stone.

Mapping Challenges And Underwater Robot Survey

Early measurements proved difficult and incomplete. Curving walls confused navigation instruments repeatedly. Positioning systems lost direction frequently. In 2017, scientists returned better prepared. They deployed a high grade underwater robot.

The robot carried advanced navigation equipment. Progress inside remained careful and slow. Eventually, researchers completed full three dimensional mapping. Multiple instruments confirmed recorded depth precisely. Each reading underwent repeated cross checking. Mapping operations lasted longer than planned.

Oxygen Loss And Chemical Isolation

Chemical changes occur below surface waters. Oxygen drops sharply after ninety metres. Above that level, limited life persists. Below it, harsh conditions dominate waters.

Hydrogen sulphide appears within deeper layers. Water circulation remains extremely limited here. Oxygen once consumed is rarely replaced. This creates stable and layered conditions.

Scientists view this isolation as valuable. Undisturbed layers preserve older ocean signals. Chemical traces reflect ancient climate conditions. Fossilised shells appear within rocky ledges. Coral fragments remain embedded in limestone.

Researchers now see the Dragon Hole differently. It represents more than former depth record. It serves as natural archive of change. Time, climate, and water shaped formation. Further study may reveal deeper history.