In the works of Plato, a Greek philosopher who lived around 2,400 years ago, the island of Atlantis was said to have been drowned in the Atlantic Ocean after the hubris of its people angered the Gods. Although Atlantis was fictional, the idea that entire civilisations can disappear beneath the waves has intrigued us for millennia. Now, innovative technologies are revealing real sites around the world that our ancestors were forced to abandon before they were swallowed by the oceans.