From PathfinderWiki

The Caltherium is a lake of black blood in the vault of Orv known as the Land of Black Blood. Physical exploration of the lake's depths is difficult because of the freezing cold nature of black blood. However, the lake also somehow prevents magical attempts to chart its depths.1

As a result, no one has ever been able to establish the actual depth of the lake, but it is at least two hundred feet deep. It covers an area of roughly a square mile, and strange waves and fogs sometimes rise from its waters. There is life in the lake – blind cave fish, frogs, and much more dangerous creatures such as chardas and (sometimes) the fearsome Moldering Emperor. It is also believed to have connections to the realms of the alghollthus.2