While Atlantis may only be a myth, there is a Japanese monument that might be the closest thing to a real-life version of the underwater kingdom. 

Dan Koh takes Neil Humphreys on a deep-sea exploration to find "Japan's Atlantis", the Yonaguni Monument that has provoked immense debate since its discovery in the 1980s. 

How was it formed? Was it man-made? Was it natural? Dan examines the evidence to come up with an answer.