A seven-mile dolphin pod has been captured by passengers on a cruise boat off the coast of San Diego, California.
The "super-mega-pod" had up to an estimated 100,000 dolphins. It was five miles wide.
Joe Dutra, captain of Hornblower Cruises, said he followed the pod for an hour. He told NBC San Diego: "When you see something like that it is truly beyond belief.
"They were coming from all directions. You could see them from as far as the eye can see.
"I've seen a lot of stuff out here but this is the biggest I've ever seen, ever. You had to be there to experience it. It was truly spectacular."
Sara Wilkin, a marine mammal expert, said the dolphins may have grouped together to make the mammoth pod because of the amount of food available in the area - there are plentiful supplies of sardines, herring and squid.
"They're attracted to the same thing, they might wind up in the same place," she said.
"They're social animals, they stick together in small groups. Sometimes, the schools come together."
Dolphins normally travel in pods of about 200. Peter Wallerstein, head of California's Marine Animal Rescue, told the Daily Mail that food had brought the animals together.
"It's all about the food," he said. "There is so much squid in the water and that is what the dolphins are chasing. The big pods are usually a little bit further offshore but they are following the food source."
Dolphins warning humans
Nicola Hodgins, of the Whale and Dolphin Conservation Society, added: "These are incredibly energetic, sociable animals. They could just be enjoying themselves [but] the likely answer is that they were following food."
A 1,000-strong dolphin pod was spotted off Dana Point in California in January.
Some observers suggested that the sight of such an enormous pod was a warning of humanity's impending doom.
They based this on Douglas Adams's space comedy classic Hitchhikers' Guide to the Galaxy, which said: "The second most intelligent creatures were dolphins who had long known of the impending destruction of the planet earth. They had made many attempts to alert mankind to the danger, but most of their communications were misinterpreted as amusing attempts to punch footballs or whistle for titbits."
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