From PathfinderWiki

Wai-gaa, also known as the "insubstantial lands", is the name given to portions of the seafloor that breakaway from the ocean's bottom and are propelled upwards by pockets of trapped gas, a phenomena unique to Xidao and the Xidao Gulf. These islands often contain strange discoveries, such as unusual aquatic beasts rarely seen at the surface, or sunken ruins. Unfortunately, the wai-gaa also sometimes dredge up drowned undead or some aquatic monstrosity. Wai-gaa are very dangerous, not just for the aquatic creatures that suddenly find themselves marooned above the waves, but also for air-breathing explorers. Wai-gaa are very unpredictable in how long they remain on the surface and they sink with incredible speed; many a sailor has meet a grizzly end when the wai-gaa they were walking on dropped back to the ocean's floor with terrifying speed.1