Sansha Yongle Blue Hole loses deepest-blue-hole record yet remains key natural archive

Key facts

• The Sansha Yongle Blue Hole, also called the Dragon Hole, sits in the South China Sea and was once confirmed as the deepest known blue hole at 301.19 metres.
• In 2017 scientists deployed a high-grade underwater robot with precise navigation tools to produce the first full three-dimensional map and confirm the depth of 301.19 metres.
• The hole’s interior bends and tilts so that its deepest point lies more than 100 metres sideways from the surface entrance.
• Rock ledges inside the hole align with ancient lower sea levels and contain fossilised shells and coral fragments.
• Oxygen nearly vanishes below 90–100 metres and hydrogen sulphide becomes detectable, leaving deeper water layers largely undisturbed.