Hard to believe that's an actual photo: it's this year's invasion of the giant jellyfish called Nomura's jellyfish. From Pink Tentacle:
This crazy photo from the Yomiuri Shimbun shows a diver swimming amongst a swarm of giant jellyfish. These giant sea blobs, known as Echizen kurage (Nomura’s jellyfish), inflict heavy damage on Japanese fisheries in the Sea of Japan each year.
This year’s invasion appears to be in full swing. The number of jellyfish has risen dramatically off the coast of Maizuru in Kyoto prefecture since Typhoon No. 13 passed over the Sea of Japan in mid-September.
Thousands of the giant jellyfish, which can grow up to 2 meters (6 ft 7 in) in diameter and weigh up to 200 kg (440 lb), become caught in fixed fishing nets each year.
Pink Tentacle Article | The Japanese, of course, dealt with this problem by eating them: Link
Previously on Neatorama: Giant Nomura's Jellyfish Attacking Asia, dated December 8, 2005.