Red Lake (Croatian: Crveno jezero) is a sinkhole containing a karst lake near the city of Imotski, Croatia. It is known for its numerous caves and remarkably high cliffs, reaching over 241 metres above normal water level and continuing below the water level. The total explored depth of this sinkhole is approximately 530 metres with a volume of roughly 25–30 million cubic meters, thus it is the third largest sinkhole in the world. Water drains out of the basin through underground waterways that descend below the level of the lake floor.
The sinkhole is named after the reddish-brown color of the surrounding cliffs, colored by iron oxides.
Like the nearby Blue Lake, it is presumed that the lake emerged when the ceiling of a large cave hall collapsed.
The lake is a habitat of the fish Delminichthys adspersus. In the dry period of the year, this fish can be occasionally seen in surrounding springs, rivers and lakes, suggesting that there is an underground connection between Red Lake and other water bodies.
At the 13th International Congress of Speleology, new findings were revealed. An inflow cave-shaped canal that measured approximately 30 x 30 meters was discovered in the eastern part of the lake at a depth of 175 meters.(Wikipedia)