Atlantis

Atlantis is a vast underwater city and respective kingdom. Atlantis was once a city covering the entirety of a small island in the Atlantic, west of the Azores. It is thought to have been founded in 3000 BC, industrialized in 1000 BC, and reached a modern level of technology in 100 BC.

In 50 BC, the island sunk in a massive earthquake-like disaster. Most of the city fell into giant chasms and pits. However, the people of Atlantis--Atlanteans--were granted the ability to breathe underwater. They rebuilt the city on the plateaus existing above the chasms and ravines, and it became greater than it ever was when it was an island city... eventually. In 100 AD it was a town of 1,000 people. In 200 AD, it was a city of 4,000 people. By 600 AD, it had 500,000 people. By 800 AD, there were 1 million people living in Atlantis, as well as 150,000 non-humans, such as cuttlefish, octopi, and squids.

To this day, the ruins of the old Atlantis still exist below the current, newer city.