There are 57 marine lakes in Palau, and each is unique, with the majority concentrated in the Rock Islands Southern Lagoon of Koror State, a UNESCO World Heritage Site. Jellyfish Lake is the only lake open to visitors. Marine lakes can be grouped into three broad categories – mixed ”holomictic,” transitional, and stratified “meromictic.” Holomictic lakes, directly connected to the ocean, have a uniform and well-oxygenated water column, and share similarities to that of the reefs found in the surrounding lagoon, often harboring corals and colorful fishes. Meromictic lakes, often found further inland with indirect connections to the ocean, have damped and delayed tides, are stratified from top to bottom, and anoxic at the bottom with poisonous hydrogen sulfide. The stratification is maintained by the reduced salinity on the surface due to high annual rainfall (~4 m per year), coupled with relatively low water exchange with the lagoon. Transitional lakes can change between being mixed or stratified, depending on weather conditions. A detailed map of Palau’s marine lakes can be found in Chapt. 10, Marine Environments of Palau.