Atlantis is a huge, ancient underwater city, inhabited by the fish-like Atlantisians who swim around the city with the fishes. It can be reached through a network of sewage pipes leading from the toilet in Edd's house. It was originally built on land before it was flooded by the sea, the cities plaque ironically reads "100% water-resistant" which nobody has been bothered to change and there are roads signs reading "road liable to flooding" which can be located around the city. Atlantis is full of rocks and boulders, as well as giant, complexly designed skyscrapers and architecture. Somewhere in the city, there is an Atlantisian caught in a tin can holder, pleading for help. All the buildings have been slightly damaged and have seaweed or algae growing on them

The city contains many different restaurants and many other tourist attractions such as Bob's Bubblepan and Don Fedit's Sushi Bar, which has an Easter Island head rested against it. Atlantis is also home to the world's biggest free-range aquarium and a museum of 'The Wonders from the Surface' which contains exhibits about life on the surface such as religion, leisure, the hall of discarded underwear and a new exhibit installed in 2008. There is also a zoo, which contains a dangerous sea monster the people of Atlantis had captured many years beforehand. A castle is located somewhere in the city where criminals are sentenced and executed, the most deadly punishment being "death by oxygen", where criminals are tied to balloons and float to the surface. The king's throne room is located in the middle of the city, where the king gives orders to the chief of police, who controls Atlantis' giant police force.

In 2008, three 'ugly strangers' from the surface visited the city in order to search for the keys one of them had accidentally flushed down the toilet. They arrived only to discover that the keys had become an exhibit in the museum of 'The Wonders from the Surface'. The three proceeded to steal the exhibit but were caught by a security camera. The King ordered the troops to be summoned and posters to be spread around the city. Despite this, the three managed to avoid to police for some time but were captured attempting to hide in the zoo when then accidently released the angry sea monster. The trio were sentenced to "death by oxygen" and floated away to the surface. However, unknown to the Atlantisians, the three strangers were actually Edd, Matt and Tom and they survived floating to the surface. It is unknown if the sea monster has been recaptured or if it is still on the loose in Atlantis, or what happened to the keys.

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