The Lions Mane Jellyfish is the largest species of jellyfish to ever be recorded.  With tentacles reaching up to 3 meters in length, each one being covered in stinging cells it's best not to get to close to one of these vast creatures.  The largest recorded Lion's Mane Jellyfish washed up on shore on Massachussets Bay in 1870 and had a body with a diameter of 7ft 6" and tentacles 120ft long (37 m).