Blue holes are among the least understood features of the ocean floor. From the surface, they can appear ordinary, even calm, blending into the surrounding reef or open water. Below, however, they drop steeply and change quickly. The Sansha Yongle Blue Hole, also known as the Dragon Hole, sits in the South China Sea and has drawn scientific attention for more than a decade. Once confirmed as the deepest known blue hole at just over 301 metres, it has since lost that record to a deeper site in Mexico. Even so, its structure, chemistry and isolation continue to make it one of the most closely studied blue holes in the world. Researchers see it less as a record holder and more as a natural archive shaped by time, water and climate.
