A diver looking for interesting undersea video footage recently got more than she bargained for off the coast of Cornwall, England, when she happened upon a giant barrel jellyfish that was bigger than she is.

"I've never seen a barrel fish — or any jellyfish — that big," diver and biologist Lizzie Daly said in a video about the dive. Clearly elated, she added, "It was the size of my body, and it was the best thing I've ever done."

In video footage of the encounter, the jellyfish looms large, pulsing along, its large frilly tentacles trailing behind it as Daly swims alongside.

Sharing a swim with the gargantuan jellyfish was "absolutely incredible," Daly said.

The giant barrel is the largest jellyfish found in U.K. waters. The Wildlife Trusts says this species of jelly, Rhizostoma pulmo, has a bell up to 3 feet wide and can weigh nearly 80 pounds. The leviathan Daly saw was clearly larger.

While she swam near it, the animal trundled along, pulsing through the water. Daly tells The Guardian that she wasn't worried about its tentacles: "It has got a very mild sting and poses no threat to humans – some people don't even feel it."

Surely, though, they would see it.

The jellyfish made such an impression on Daly, a conservation advocate and wildlife host who works with the BBC and other outlets, that on Tuesday she wrote a thank-you note to her new rhizostomatid friend.

"Dear Giant Jellyfish," Day wrote. "Do you know how many people you have inspired in the last few days? People asking..what is it? Is it really that big? That cannot be in UK waters?!!"

Daly's swim with the jellyfish was captured on video by underwater camera operator Dan Abbott — who captioned his photo of Daly being dwarfed by the jellyfish, " I can't believe that just happened!"

Daly and Abbott were working on a special called Wild Ocean Week, highlighting the splendor that can be explored underwater, and to encourage people and organizations to do more to preserve the oceans' beauty.

Saying her goal was to inspire a wider audience, Daly wrote, "Giant jellyfish you have done just that. So THANK YOU!! I feel humbled to have shared the same space as you."

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