The divers, Lizzie Daly, a biologist, and Dan Abbott, an underwater cinematographer, per Live Science, captured the event on camera. Daly later took to Facebook with footage of the event.
The pair, who were diving off the coast of Cornwall on Saturday, saw the giant creature after it “appeared out of the murky water,” Daly told Vice. They then swam with the jellyfish for about an hour.
“It really humbles you to be alongside an animal that size,” Daly added. “It’s an experience we’ll never forget.”
“My first reaction was that I’ve never seen a jellyfish that size in my life,” Abbott told The Washington Post of the “beautiful, majestic, slow-moving and graceful animal.”
Barrel jellyfish are “gentle giants” that “swarm in warmer summers when there are large plankton blooms offering a plentiful food supply,” according to Buglife.
“Their soft bodies are over 90 [percent] water, and move in a mesmerizing pulsing motion operated by a simple net of nerves. Though infamous for their stinging tentacles, most species are harmless to humans,” it added. The barrel jellyfish is the largest found in U.K. waters, per the site.