Last weekend, two divers exploring the waters off the coast of Cornwall, England, had a surreal encounter with a roughly 5-foot-long barrel jellyfish.

As Lizzie Daly, a biologist and ambassador for the United Kingdom’s Marine Conservation Society, tells USA Today’s Ryan W. Miller, she and cameraman Dan Abbott were exploring the area as part of a fundraising and awareness campaign dubbed Wild Ocean Week.

Daly and Abbott had been diving for about half an hour when a “single enormous jellyfish” unlike any they had ever seen before emerged out of the murky water. The pair spent the next hour or so swimming alongside the unusual specimen, recording footage they later condensed into a two-minute video posted on Facebook.

“It really humbles you to be alongside an animal that size,” Daly says in an interview with Vice’s Madeleine Gregory. “It’s an experience we’ll never forget.”

According to the Guardian’s Mattha Busby, barrel jellyfish—the largest of their kind in the U.K., the animals boast eight frilly arms equipped with stinging tentacles and typically grow to a length of up to 3 feet—spend their summers in the warm waters off the country’s western coast. Few humans meet members of the species in their natural habitat; instead, most people encounter the marine animals when they wash up on beaches. (Per Live Science’s Brandon Specktor, barrel jellyfish pale in comparison with the lion’s mane jellyfish, which is known to reach a length of some 120 feet, or longer than the average blue whale.)

Speaking with Morgan Krakow of the Washington Post, Cheryl Ames, a research collaborator at the Smithsonian’s National Museum of Natural History who was not involved in producing the viral video, explains that barrel jellyfish begin life as tiny, 1-millimeter-long creatures. In captivity, they are unable to reach the staggering size seen by the divers, but in the wild (the animal’s natural habitat ranges from the U.K. coast to the Mediterranean), they can grow largely unchecked.

As Daly tells Busby, she and Abbott felt comfortable around the jellyfish because it has a “very mild sting and poses no threat to humans.” In fact, the biologist adds, some people don’t even feel the animal’s sting.

“Many people would be immediately worried, but it is not dangerous,” Daly says. “It’s a majestic creature.”

In an Instagram post, Daly described Wild Ocean Week as an “ambitious journey around the U.K. to document as many encounters with our charismatic marine species as possible.”

Reflecting on the week-long expedition, Daly tells CBS News’ Caitlin O’Kane that she and Abbott spotted animals ranging from seals to breaching minke whales, lunge-feeding whales, dolphins and—finally—the giant jellyfish.

The ethereal footage of this meeting “is the one that has inspired the most people,” Daly concludes to Vice’s Gregory. “The goal was connecting people to nature, and the barrel jellyfish did that for sure.”