The Sunken City (水没ビル, Suibotsu Biru, Submerged Building) is an area accessible from Monochrome GB World's dungeon.
The Sunken City is an area containing a large expanse of water. The remains of tall skyscrapers stick out of the water's surface, which has completely submerged any smaller structures below.
It constantly rains in this area, and using the Teru Teru Bōzu effect here will make it rain even harder. Using it a second time will stop the rain completely.
The portal from Monochrome GB World's dungeon places you on the roof of a partially-submerged building with a ladder on one side. At the bottom of the ladder is a boat that Urotsuki can ride in.
Once Urotsuki steps on this boat, she cannot immediately return to the portal leading back to Monochrome GB World, as climbing back up the ladder will instead take Urotsuki to a huge complex with a series of ladders running up the side. (She can return, however, by going deeper into the Buried City and taking the UFO there) At the very top of the complex, there is a set of footprints on the edge of the roof. Walking onto them will darken Urotsuki's expression, and she will jump off the edge. After the fall, she will wake up on the floor in the lower reef area of Atlantis.
About halfway up the group of buildings, there is a long ladder on the far right side. Going down it will take you to a lower rooftop area with a Vending Machine and a door leading into a long, dark passage. At the end of the passage is a doorway, which leads Urotsuki to the Buried City.