Underwater with Sri Lanka's sperm whales – in pictures

The sperm whale may be one of the most successful animals in the ocean, boasting a global distribution that survived the toll of the 20th century, when whaling factory fleets took three million great whales from the seas. Now a newly identified population in the Indian Ocean is attracting the attention of scientists, conservationists - and soon, tourists, too. How will this whale weather the new storm of attention? Underwater photographer Andrew Sutton gained special access to the gentle giants that swim around the island of Sri Lanka. Words by author Philip Hoare

Every March, vast numbers of sperm whales gather in the deep waters north-west of Sri Lanka. Andrew Sutton's photographs are vivid evidence of a little-known population - all the more surprising since sperm whales are the largest active predators on the planet with males reaching nearly 20m (65ft) in length. As natural submarines, they shut down all their organs except for their heart and brain, and using their muscular tails are able to dive for up to a mile, spending up to two hours feeding on squid.