Atlanta was a city on the planet Earth that was once landlocked. However, in an effort to boost tourism and become a bigger Delta hub, it was moved offshore. The city then subsequently overdeveloped causing itself to sink to the ocean bottom and its inhabitants evolved into mer-people more quickly than usual since the caffeine from the Coca-Cola plant helped speed things up.
The city was rediscovered by the Planet Express crew when they went on a fishing trip in the middle of the ocean to fulfill the terms of Hermes's mandatory fishing license.