Millions of golden jellyfish pulsing through green, sunlit water: few swims on Earth feel as strange as a visit to Jellyfish Lake in Palau. Photographer Nana Trongratanawong captured that feeling with a GoPro while freediving among the animals, and the film, colour-graded and shared by GoPro in May 2014, drew more than 150,000 views in its first two days.

Aerial view of forest-ringed Jellyfish Lake on Eil Malk, Palau
Jellyfish Lake on Eil Malk, ringed by the forest of Palau’s Rock Islands.

Where Jellyfish Lake is: Eil Malk in Palau’s Rock Islands

Jellyfish Lake, known in Palauan as Ongeim’l Tketau, lies on Eil Malk, one of the forested limestone Rock Islands in the Southern Lagoon of the Republic of Palau, between Koror and Peleliu. The lake is about 460 metres long and 160 metres wide, covers roughly 5.7 hectares and averages about 30 metres deep.

It formed around 12,000 years ago, when rising seas after the last ice age filled a basin inside the island. The lake still has indirect connections to the lagoon through cracks and tunnels in the limestone, so it rises and falls with the tide, about two hours later than the sea outside and with less than half the range.

The golden jellyfish and their daily journey

The lake’s famous residents are golden jellyfish, Mastigias papua etpisoni, a subspecies found only here and named after former Palauan president Ngiratkel Etpison. In a typical year around five million of them live in the lake.

Their colour comes from zooxanthellae, single-celled algae living inside their tissues. The algae need sunlight, and in return they feed the jellyfish with energy and nutrients. That partnership drives a remarkable daily migration:

  • At sunrise the jellyfish swim east, towards the morning light.
  • They stop at the shadow line cast by the trees and cliffs around the shore, where a dense “wall” of jellyfish can form on sunny mornings.
  • Around midday they turn and swim west, again halting at the edge of the shade.

Staying out of the shadows also keeps them away from the anemones on the lake’s edges, which prey on jellyfish. The golden jellyfish do have stinging cells, which they use to catch zooplankton, but the sting is so mild that it is essentially harmless to people.

A lake with two layers of water

Jellyfish Lake is meromictic, meaning its layers of water never mix. The upper layer, from the surface to roughly 15 metres, holds oxygen and all the life visitors see. At 13 to 15 metres a pink layer of bacteria marks the boundary. Below it there is no light and no oxygen, and the water carries high concentrations of hydrogen sulfide, which can be absorbed through the skin and is poisonous.

This is why scuba diving is prohibited in the lake. Tank bubbles can also injure the fragile jellyfish. Snorkellers and freedivers who stay in the upper water, as Trongratanawong did, are safe from the deep layer; the lake’s researchers advise staying above 12 metres.

El Niño and the changing jellyfish population

The jellyfish depend on their algae, and the algae are sensitive to heat. During strong El Niño events the lake warms and the population can crash:

  • In late 1998 the medusae disappeared entirely after an El Niño raised water temperatures. They reappeared in January 2000 and returned to their earlier numbers by 2012.
  • After the 2015–16 El Niño drought the population dwindled again, and the last medusa was seen in May 2016.
  • The lake began to cool in late 2018, and by 2019 the population was once more estimated in the millions.

The Coral Reef Research Foundation in Koror monitors the lake and publishes a current conditions and forecast page, which is the place to check before you book a trip around the swim. Its Jellyfish Lake guide explains the science in more detail.

Visiting Jellyfish Lake: permits, access and etiquette

Jellyfish Lake is the only one of Palau’s marine lakes open to tourists. Day trips leave from Koror with local tour operators; the boat ride to Eil Malk takes about 45 minutes, followed by a short trail from the beach up to the lake. Visitors need a Rock Islands and Jellyfish Lake permit, which Wikipedia lists at US$100 and valid for five days.

A few simple habits protect the lake:

  • Empty your pockets and bring nothing from the ocean into the lake, not even shells or rocks. An invasive anemone may have arrived this way.
  • Use reef-safe sunscreen and apply it about 30 minutes before you swim.
  • Kick gently and keep fins low. Trongratanawong herself described the jellyfish as harmless but fragile.
  • From October to March the jellyfish tend to gather nearer the lake’s south edge.

Combine the swim with the rest of the Rock Islands, a UNESCO World Heritage landscape of lagoons, reefs and mushroom-shaped islets; our Palau marine-lakes guide covers them. For another layered body of water, see Cenote Angelita, and for a classic reef stay, Dunbar Rock in Honduras.

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