Sansha Yongle Blue Hole Reveals Complex Ocean History

Blue holes are among the least understood features of the ocean floor. From the surface they appear calm, merging with nearby reefs or open sea. Beneath, they descend sharply and shift in form and chemistry. The Sansha Yongle Blue Hole, also called the Dragon Hole, in the South China Sea has drawn sustained scientific interest. Once confirmed as the world’s deepest blue hole at 301.19 metres, it has since lost that record to a deeper site in Mexico. Yet its structure and isolation continue to make it a vital site for marine research.

Complex Three-Dimensional Structure

Detailed surveys show that the Dragon Hole is not a straight vertical shaft. It bends and tilts as it descends. The deepest point lies more than 100 metres horizontally from the surface opening. The entrance is broad near the top, then narrows sharply before widening again 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 Exams

• Blue 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 life 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.