SAN DIEGO — A school of wild California anchovies was so big it rivals anything seen off the San Diego coast in three decades. And it was spotted by researchers.

“It was an amazing experience,” said Jenn Moffatt of Birch Aquarium. “Thousands and thousands of fish. And when you dive down into the school, they respond to you, they swim off in all sorts of directions, they’ll swim around you, behind you.  You can’t see where they start and where they end.”

Moffatt was one of the lucky ones. She swam with the fishes and video shot by snorkelers from the Scripps Institution of Oceanography saw the same thing she did.

The video shows millions of anchovies.

“We could see schools extending all the way from the point in La Jolla to as far as you could see to the north of us,” Moffatt said.

The black band of fish extended for several miles and extended an estimated 15 feet deep and 100 meters wide. Although it’s not unusual for northern anchovies to swim in schools, a school this big is unheard of, especially this far south.

“So it’s not really clear if the high water temperature affected their behavior, causing them to come into shore or not,” Moffatt said. “It seems more likely that warmer water would force them off shore, so it’s really kind of a mystery as to why they’ve aggregated here in such massive numbers.”

Some divers who swam below the school said it was a complete blackout above.

They were the lucky ones because almost as quickly as the massive school of anchovies formed, it also disappeared.