Sansha Yongle Blue Hole Reveals Complex Ocean History

 



at certain depths. This uneven morphology reflects gradual formation over time rather than a single collapse event. Reef limestone dominates the walls, shaped by long-term erosion and sea-level change.

Advanced Mapping and Measurement

Early measurement attempts were hindered by the hole’s twisting interior. Navigation systems struggled with angled walls and shifting passages. In 2017, researchers deployed a high-grade remotely operated vehicle fitted with precise positioning tools. The mission produced the first full three-dimensional map. Multiple instruments were cross-checked to ensure accuracy. The final confirmed depth was recorded at 301.19 metres with minimal uncertainty. The mapping process took longer than expected due to the hole’s complex geometry.

Geological Clues from Stone Ledges

Rock formations inside the hole reveal signs of earlier shorelines. Step-like ledges align with periods when global sea levels were lower during colder climatic phases. These features suggest the cavity expanded in stages as sea levels rose and fell over thousands of years. Side cavities and abrupt openings further indicate prolonged interaction between rock, seawater and climatic shifts.

Important Facts for ExamsBlue holes are underwater sinkholes formed mainly in carbonate rocks such as limestone.

The Sansha Yongle Blue Hole lies in the South China Sea and measures 301.19 metres in depth.
It was once the deepest known blue hole before a deeper site was confirmed in Mexico.
Such formations help reconstruct past sea-level and climate changes.

Sharp Chemical Stratification Below 100 Metres

One of the most striking features is the rapid loss of oxygen below 90 to 100 metres. Above this layer, limited marine lfe survives. Below it, oxygen is nearly absent and hydrogen sulphide becomes detectable. Water circulation is extremely weak, preventing mixing with surrounding seawater. This stable layering preserves chemical signals from past ocean conditions, turning the Dragon Hole into a natural archive of environmental history.


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