Jellyfish Lake, Eil Malk Island, Palau, Pacific Ocean

The anoxic layer extends from about 15 metres (49 ft) below the surface to the bottom of the lake. The oxygen concentration in this layer is zero. The hydrogen sulfide concentration rises from about zero at the top of this layer to over 80 mg/liter at the bottom of the lake. The top three meters of this layer contains a dense population of bacteria, at least one species of which is a purple photosynthetic sulfur bacterium. This bacterial layer absorbs all sunlight so that the anoxic layer below the bacterial plate is dark, but transparent. Hamner estimated the visibility to be about 30 meters. The anoxic layer also contains high concentrations of ammonia and phosphate which are almost completely absent from the upper layer. The anoxic layer is potentially dangerous for divers, who can be poisoned through their skin. This risk is mitigated as scuba diving equipment is not allowed in the lake, thus limiting the depths to which individuals may dive.
Age of the lake
Jellyfish Lake is around 12,000 years old. This age estimate is based on the depth of the lake (about 30 meters), an estimate of the thickness of the sediment (at least 20 meters) and the rising sea level since the end of the last ice age. About 12,000 years ago, the sea level had risen to the point that sea water began to fill the Jellyfish Lake basin.