Picture yourself diving through a serene body of water in tropical Indonesia. Rays of sunlight penetrate the surface of the water, heating and brightening it up. As you breaststroke through the salty surf, gelatinous, tentacled creatures begin popping up all around you, their pale-yellow bodies contrasting with the bright blue of the water.

Suddenly, you're swimming alongside countless semi-translucent, coffee mug-size golden jellyfish. Unlike some larger, more venomous species, these jellies can't hurt you. Golden jellyfish, like moon jellies, have a sting so minor humans can't feel it.

There are about 200 marine lakes, or small, landlocked bodies of seawater, known to science. Of those lakes, only a handful contain jellyfish, like famous Jellyfish Lake in Eil Malk. Often found in Palau, Vietnam, and Indonesia, the remote locations of these lakes make them difficult for researchers to study. (Read about why Jellyfish Lake is running out of jellyfish.)

But with the help of the National Geographic Society, marine biologist Lisa Becking did just that. From her base at Wageningen University and Research in the Netherlands, Becking gathered up a team of like-minded researchers to travel to the Raja Ampat islands in West Papua, Indonesia. On expeditions in 2011 and 2016, they studied the lakes, documenting biodiversity and mapping out ideas for conservation management.

Swimming With Jellies

Marine lakes are formed in karstic landscapes that have been carved out of eroded limestone. The area of the expedition has plentiful karst deposits, making it prime real estate for marine exploration.

Between trekking and gathering samples, Becking and her team filmed two hours of aerial footage of local marine lakes. Then, after combining that footage with Google maps and old Dutch maps, they found 42 marine lakes that had not been documented before.

"They're kind of like island systems," Becking says, referring to how salty marine lakes are isolated from the rest of the sea. "Islands play a crucial role in biology generally because they're an ideal setting for studying how biodiversity is formed. They're kind of like these giant natural experiments."The team visited 13 of these lakes during the expeditions and found that four contained jellyfish.

Protected from winds by surrounding cliffs, the lakes are meromictic, meaning they're composed of layers that don't intermix. They're about 65 feet deep, and they have an anoxic layer about 26 feet down, where the pitch-black water is oxygen-depleted and filled with bacteria. Below that is a layer of hydrogen sulfide, which is dangerous to human divers.

Up at the top, the waters are shimmering with colorful golden jellyfish and moon jellies. Throughout the day, these species make daily migrations to follow the sun, congregating tightly in the sun-filled parts and avoiding the shaded sections. (Related: "Blue Herons, Jellyfish, and Other Animals With Daily Commutes")

Endless Jellyfish Migration One of Palau's salt water lake is home to millions of trapped jellyfish that migrate endlessly to cultivate an algae.

"This surface is just bubbling with the [grown] medusa," Becking says. "It's quite a colorful event."

Beyond the jellyfish themselves, more than a hundred species of sponges inhabit the lakes' coasts, colored in shades of purple, pink, blue, and yellow. Becking and her team discovered at least four new species during their expeditions. (Related: "The Surprisingly Complicated Construction Work of Simple Sponges")

Becking swam alongside jellyfish before these expeditions, and she recalls the experiences as frightening and counterintuitive—we are taught from a young age to avoid jellyfish because of their hurtful stings. But with their harmless jellies, these lakes are actually quite the tourist destination because they allow divers to swim alongside the creatures without the threat of being stung. Since that first swim, Becking has bopped with jellyfish many times. (Related: Watch a sea turtle eat a jellyfish like spaghetti.)

"They move on their own and they're just sort of bumping against you all the time," Becking says. She adds that the jellies tend to hover around her water-proof notepads when she's jotting down notes, "very much like a cat, although I don't think they're actually asking for attention like cats do."

The next step for Becking and her team is to go back into satellite images from the 1970s onward. From that data, they'll look at the color of the lakes—an indication of if there are jellyfish or not—and see how those shades change over time and why. They may be able to link this data with changing temperatures.

"There's still a lot more to explore in the sea," Becking says, "and I do think that kind of exploratory work does lead to a better understanding of the ecosystem."

Preserving Blooms

The ecosystems that marine lakes host are fragile. Isolated but still maintaining subterranean connections to the sea, the lakes are particularly susceptible to climate change. Their waters are warmer and saltier than the open ocean, which provides a sneak peek into how oceans might fare if climate change continues at its current rate. (Related: "Climate Change May Shrink the World's Fish")

This Jellyfish Contains an Ecosystem Many species of amphipods need to live upon jellyfish for at least some of their lives.

Jellyfish have lived in the ocean longer than any other macroscopic creature, and climate change could actually be to their advantage, because they tend to thrive in warmer waters. But not all marine lakes have medusas, the bell-shape organisms you probably think of when the word "jellyfish" comes to mind. Some lakes have young polyps that hang out on the lake bottom, or reduced populations all together. Salinity and warm temperatures might also influence jellyfish numbers.

These warm conditions also make for unique species. Of all the species the team sampled, at least a quarter of them seem to be endemic.

When Becking and her team traveled to the area, they had to coordinate their own travel. Now, there's a ferry that goes out to the islands twice each week, making them more accessible to tourists. The lakes are often used as aquaculture ponds, and are plagued by invasive species and litter.

The team has presented its findings to local and regional governments, and now they're working together on conservation management for the area. On a local and regional level, they plan to carry out monitoring efforts next month.