Atlantis

Aeralanteris, or Atlantis, is a undersea kingdom located on the southern edge of the fertile reef. Picaria, the capital of Atlantis, is a large city. However, due to the immense pressures at the seafloor, tall structures cannot be built well. Therefore, a extremely large dome was formed around Picaria, which keeps the atmosphere pressure at around the same as sealevel. The merpeople of Atlantis can survive low pressures, but humanoids cannot survive high pressures.

Since merpeople can only swim in water, the Picarian dome is filled with H2O molecules that are dispersed enough so that the pressure at the top of the dome is 0.6 atm and the bottom 1.4 atm. Merpeople can swim (albeit with difficulty), and humanoids only need air to survive.