The pod was more than 7 miles long and 5 miles wide, according to Capt. Joe Dutra of Hornblower Cruises, who spotted the dolphins while sailing his daily tour.
Dutra told NBC San Diego he's never seen anything like it.
“When you see something that is honestly truly beyond belief,” Dutra said.
Dutra estimated the pod may have been as large as 100,000 dolphins.
They were coming from all directions, you could see them from as far as the eye can see,” he said. “I’ve seen a lot of stuff out here… but this is the biggest I’ve ever seen, ever.”
Dolphins usually travel in groups of less than 200, but marine mammal expert Sarah Wilkin told NBC the right food source, such as an abundance of squid, herring and sardines, could get such a large number of the animals in the same place.
"They’re definitely social animals, they stick together in small groups,” she said. “But sometimes, the schools come together.”