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.
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".
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.