The Third City is the underwater city of the Maerum that April is taken to after the sinking of the White Dragon nearby. Distinctive for its fantastic tall spires, it is located near to the island of Alais in the Sea of Songs, not far from the reefs.
Presumably home to both Maerum and their human gatherers, the Third City is ruled over by the Maerum Queen; the Waterstiller's spear is kept in the palace there. Just outside the city near the Landwalker's Bubble is the Ancient Shrine where the Maerum's piece of the Stone Disk is to be found, along with the murals that show the history of the Maerum and Alatien, and somewhere close by is the deep abyss where the Old God is sleeping.
Its name could indicate either that it is the third of three or more current Maerum cities, or that it is the third city they have had to build after the destruction of previous ones.