Marine expert Matt Slater, with his dog Mango, has swam with a 20kilo barrel jellyfish in the Percuil Estuary near St Mawes.
He captured this incredible footage:
Many people will have seen these enormous creatures in the water as this has been a particularly good year for them.
Matt has described his experience as 'otherworldly'. Despite their size, barrel jellyfish are harmless to humans.
– Matt Slater, Marine Awareness Officer
These creatures are incredibly beautiful when you get a close look at them.
The tentacles really look like soft coral, and round the edge of the jellyfish’s umbrella like bell there is a deep blue line punctuated every twenty centimetres or so with a tiny dot, a sensory statocyst.
Jellies are more aware of the watery world around them than you may imagine. They are constantly swimming up and down in the water column looking for profitable patches of plankton.
The statocysts are their sensory cells that enable them to orientate and tell up from down.
The increase in jellyfish numbers around Cornwall this year is down to an increase in their food source. Matt says this is the "largest number of jellyfish seen in the waters here since 2002".
Lots of people have been calling Cornwall Wildlife Trust to ask us why there are so many jellies this year.
In the spring tiny anemone like jellyfish ‘polyps’ living on the sea bed expand in size and then bud off thousands of tiny larvae.
Most years these larvae will perish but in years where the conditions are good, temperatures are optimal, there is plenty of planktonic food and predators do not eat them all, large numbers of them will survive creating these huge jellyfish swarms.
It is a boom and bust cycle.
Cornwall's north coast has also seen huge numbers of blue jellyfish. Unlike barrel jellyfish, these can give a mild sting.
Matt has also reported sightings of compass jellyfish and moon jellyfish in Cornish waters. The Cornwall Wildlife Trust is urging people to get in touch and record their sightings.