An eruption of Santorini some 3,600 years ago was one of the largest, yet most mysterious, natural calamities in human history and may have wiped out an entire civilization. Much of the evidence of its destruction has been lost at sea until now. National Geographic Explorer-in-Residence Robert Ballard and an international team of volcanologists, armed with a state-of-the-art Remote Operated Vehicle fitted with HD cameras, dive into the belly of the Santorini's submerged caldera.