Jellyfish Lake, Eil Malk Island, Palau, Pacific Ocean

The lake is connected to the sea via three tunnels that lie near the surface. The tunnels channel tidal water in and out of the lake. Tide levels in the lake are damped to about a third of the lagoon tidal levels. The tidal peaks are delayed from the lagoon tidal peaks by about 1 hour and forty minutes. Biologist William Hamner estimated that about 2.5% of the lake's volume is exchanged during a tidal flow. However because the tidal water enters at the surface, the lower anoxic layer is largely unaffected by tidal flows.
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.