A pair of divers swimming off the southwestern tip of England hit the jelly jackpot last week after crossing paths with a hulking barrel jellyfish (Rhizostoma pulmo) — a rarely seen species that can grow about as large as a full-grown human. Luckily, they filmed the whole thing.

The divers — biologist Lizzie Daly and underwater cinematographer Dan Abbott — shared the encounter in a Facebook video posted Saturday (July 13) as part of the Wild Ocean Week campaign — a series of videos showcasing the oddities of the deep to help raise money for the United Kingdom's Marine Conservation Society.

Daly and Abbott were diving off the coast of Cornwall, U.K., when they saw the giant jelly emerge from the murky water. Also called the dustbin-lid jellyfish, the species is characterized by eight puffy arms capped by stinging tentacles and a large, globular head that lends the creature its unglamorous nickname. Barrel jellyfish sometimes wash up on the shore, Daly told Vice, but it's rare for a diver to swim face-to-faceless-head with one of the massive blobs. [Marine Marvels: Spectacular Photos of Sea Creatures]

While the barrel jellyfish is the largest species of jelly found in U.K. waters, it's a mere shrimp when compared to the lion's mane jellyfish (Cyanea capillata), the largest known species in the world. This coldwater jelly is known for its galaxy of 1,200 long, trailing tentacles, which can bring an individual creature's total body length to up to 120 feet (36.5 meters) — which is longer than the average blue whale.

This tangle of tentacles is so massive that a single lion's mane jelly may be able to sting 50 to 100 people in just a few minutes if currents carry the jelly too close to a populated coast — a fun fact that a group of unlucky New Hampshire beachgoers learned the hard way in 2010.

Dangers in the Deep: 10 Scariest Sea CreaturesIn Photos: Spooky Deep-Sea Creatures15 of the Largest Animals of Their Kind on EarthOriginally published on Live Science.