2009 年，一群英国洞穴专家到越南中部丛林深处探险。他们惊讶地发现一个非常巨大的洞穴，并认为这是全球最大的洞穴。这班专家是首度进入此洞穴的人类，他们在地底下走了6公里，直到被巨大岩层挡住，他们称之为「越南长城」。现在他们带著攀岩装备回来，连同地质学家和动物学家，一起研究这是否真的是全球最大洞穴，以及洞穴里隐藏了什么祕密？ In 2009, a team of British cavers investigated a recently uncovered cave in a remote Vietnamese jungle. The Son Doong cave is enormous; can it be larger than the current world-record holder? The explorers traveled for miles through the cave before hitting a 46-foot-high wall. Now, follow the team as they return to Son Doong to finish exploring the cave and climb the wall. Will Son Doong prove to be the worlds biggest cave? And what secrets are undiscovered inside this unique ecosystem?