Diver Shawn Stamback and his friends were hoping to hear a whale song while snorkelling off the coast of California when a mass of sardines swam past and two humpbacks burst to the surface.
Stamback thought he would be crushed by the whales' massive tails.
"All these bait fish, just coming straight up in your face and ... two humpback whales were right behind him. Chasing them all the way to the surface," he said.
"I worked my way over to the boat. Got up on the back of it as soon as I could."
Vancouver whale researcher Chad Nordstrom said the whales were using a predatory technique called lunge feeding.
"They circle down below a bait ball — a group of small schooling fish," he said.
"And they come up below the animals and use those huge throat pleats that they have to engulf a whole series of water and fish all at the same time."
Nordstrom said he's seen whales use this feeding technique off the B.C. coast, but usually from a distance.
"It's the first time I have ever heard of or seen anyone that close while they were in the water."
Guidelines suggest being 100 metres away from humpback whales. Nordstrom believes the divers were lucky to survive.
But for Stamback, he now has a whale of a tale to tell.
"The other guys said they had never seen me swim that fast before, and I can see why now that I have watched the video."