Jellyfish Lake

Jellyfish Lake (Palauan Ongeim'l Tketau, "Fifth Lake") is a marine lake located on Eil Malk island in Palau. Eil Malk is part of the Rock Islands

, a group of small, rocky, mostly uninhabited islands in Palau's Southern Lagoon, between Koror and Peleliu. There are about 70 other marine lakes located throughout the Rock Islands. Jellyfish Lake is one of Palau's most famous dive (snorkeling only) sites. It is notable for the millions of golden jellyfish which migrate horizontally across the lake daily.
Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of ancient Miocene reef. However the lake is sufficiently isolated and the conditions are different enough that the diversity of species in the lake is greatly reduced from the nearby lagoon. The golden jellyfish Mastigias cf. papua etpisoni and possibly other species in the lake have evolved to be substantially different from their close relatives living in the nearby lagoons.

Snorkeling in Jellyfish Lake is a popular activity for tourists to Palau. Several tour operators in Koror offer trips to the lake. Eil Malk island is approximately a 45 minute boat ride from Koror. The lake is accessed by a short trail from the beach on Eil Malk to the lake.
Tourists require a pass to access to Jellyfish Lake. The Rock Islands/Jellyfish Lake pass is $35 and is good for 10 days.
Scuba diving by tourists in the lake is not allowed. Two reasons are put forward for this:
The bubbles from scuba tanks can harm the jellyfish if they collect beneath their bell.
The anoxic layer that begins at about 15 meters contains high concentrations of hydrogen sulfide which can be absorbed through the skin of a diver which can lead to death.
Jellyfish Lake is currently the only one of Palau's marine lakes open to tourists.