Three-Dimensional Morphology of Sansha Yongle Blue Hole

The Sansha Yongle Blue Hole is the deepest known marine blue hole in the world, and its 3D geometric parameters and fine morphology are closely related to the formation and evolution of the Blue Hole. In 2017, ROV and a multibeam echosounding system were used to survey the Blue Hole, and fine water depth and 3D morphological data of the hole were obtained. In this study, the point cloud data obtained during the surveying period are used to establish a detailed 3D model for the Blue Hole, calculate the geometric parameters of the Blue Hole, and analyze the 3D morphological characteristics of the Blue Hole. The depth of the bottom of the Blue Hole is calculated to be 301.19 m based on the local ten-year average sea level, the volume is 1 417 149 m³, and the surface area is 98151 m². Its morphology is complex and twisted, and the hole plane is generally spreading northeastwards, and the sharp corner of the hole bottom deviates from its entrance by 118 m, and the overall appearance resembles a sport ballet dancer’s shoe. The hole body can be subdivided into five layers in the vertical direction as follows. Cave Ⅰ is the cave entrance part, showing a shallow disk mouth shape with a gentle slope; cave Ⅱ and Ⅲ show a nearly cylindrical body with vertical descent and uniform width; cave Ⅳ has an extremely complex form, and its volume is the largest, and the form turns northward twice at 90 m and 158 m; cave Ⅴ extends to the northeast, and the diameter of the cave gradually becomes smaller, forming a bowl-shaped cave bottom with an average width of about 36 m. The inner wall of the cave is uneven, and there are small morphological features such as nests, protrusions, corrugations, side holes, lateral grooves, steps and longitudinal fractures, which show the complexity of the internal morphology of Blue Hole. The overall complex morphology and the small morphological features with different internal morphological characteristics of Blue Hole provide some reliable evidences for its evolution and genesis.