Dan Abbott and Lizzie Daly were diving in the Celtic Sea when they swam into a monster of a sea creature – the giant barrel jellyfish (Rhizostoma pulmo) cruising along the Cornish coast, in the UK, was roughly the size of a singular human.

Fortunately for us, the encounter was captured on film by Abbott, an underwater cinematographer, while wildlife biologist and presenter Daly swam alongside the jellyfish to put the size of the creature into perspective.

The giant barrel jellyfish is the largest species of jellyfish to visit British waters and can grow up to 90 centimeters (2.9 feet) wide – making it roughly equivalent in size to a dustbin (UK-speak for trash can) lid. Hence their nickname, the dustbin-lid jellyfish. Add to that their arms, which can reach lengths of 1.9 meters (or 6 feet), and the animal makes for quite an impressive sight. 

The jellyfish's tentacles are found on each of their eight arms. These arms have a translucent and frilly appearance and – luckily for Abbott, Daly, and any other keen diver – the tentacles' sting is not especially harmful to humans. (Medical experts do still advise giving the tentacles a wide birth.)

In general, you will only come into contact with a giant barrel jellyfish if you are out at sea. But there have been instances of jellyfish coming closer to shore, for example, in 2002, 2014, and 2019. It's a scene that tends to succeed a mild winter and subsequent plankton bloom, which the jellyfish feast on.

Believe it or not, despite its impressive size the giant barrel jellyfish is not the largest species of jellyfish in the world. That title most likely goes to the lion’s mane jellyfish (Cyanea capillata), which has been known to reach lengths of 36.5 meters (120 feet) from the top of the body to the bottom of the tentacles, putting it in the same league as the blue whale (up to 32 meters or 105 feet) in terms of length. 

The lion's mane jellyfish (so-called because of its long, thin, hair-like tentacles) can be found in the Arctic and North Pacific Ocean from Alaska to Washington and is more dangerous than the giant barrel jellyfish. Their tentacles contain large volumes of neurotoxins that can result in anything from a rash to respiratory problems. 

As for Abbott, Davy, and the giant barrel jellyfish, the video was taken on the last day of Wild Ocean Week (8 July - 14 July), an initiative Davy launched to raise awareness and money for the UK-based Marine Conservation Society.

"What an INCREDIBLE experience – both Dan and I have never seen anything like it. I couldn't think of a better way to finish the week in celebrating our incredible oceans," Davy said in a Facebook post.