It looks like a regular bubble and is kept in a glass container. It was one of Atlantis's most prized possessions, being one million years old.
Role in episode
SpongeBob and Patrick want to see the oldest living bubble when coming to Atlantis. Patrick takes a picture of the bubble and it then pops. They later find out that the bubble they popped was not the actual bubble and was a replica.
Patrick takes a picture of the real bubble and it pops. The bubble is later replaced with Plankton.