From PathfinderWiki

Aya-Maru is the name of a deep underwater trench that almost splits the centre of the aquatic nation of Xidao. While Xidao is a nation of relatively pacifistic locathahs, cecaelias and tritons, the Aya-Maru is the reason why it has never known true peace. While most of the Xidao Gulf is only a few hundred feet deep, the Aya-Maru is over three miles deep with some claiming it has no bottom, instead linking to some deep underwater region of Hell itself. The walls of Aya-Maru are riddled with caves and cavern complexes that serve as the lairs for countless tribes and petty kingdoms of sahuagin and merrows. Were these bickering groups ever to be united under a single leader the devastation that they could cause Xidao and the land-bound kingdoms surrounding it would be apocalyptic. But something even worse lurks below this, at the deepest points of the Aya-Maru dwell the krakens. Mercifully these vast, powerful, malevolent beings almost never visit the surface and those that do are normally outcasts or exiles, banished from Aya-Maru's benighted depths.1