China’s massive Yulin Naval Base on Hainan Island is one of the greatest strategic interests in the region. It is home to China’s nuclear ballistic missile submarine fleet—the backbone of its second-strike deterrent—as well as other submarines, the Drive reports. It sits at the northern edge of the highly contentious South China Sea. To its east is the gateway to the open Pacific and Taiwan. The most intriguing feature of this facility is the mysterious submarine cave built into the side of a mountain that dominates the southern end of the installation. Although I have seen satellite images of the roadway barges removed from the opening, we have never seen one with a submarine actually using it, until now.