Kakaban Lake, though, is not the only place in Indonesia to swim, pain free, with jellyfish.

Researchers have found at least half a dozen marine lakes where jellyfish have evolved in similar ways. At least two can be found in the Raja Ampat islands of Indonesia, more than 600 miles southeast of Kakaban Lake.

The world’s best-known jellyfish lake, in the nation of Palau, suffered a dramatic population crash in 2016, most likely because of drought and increased salinity caused by an El Niño weather condition. The deaths underscored how vulnerable the animals are to fluctuations in their environment.

While the jellyfish continue to thrive on Kakaban, the island has just two human inhabitants, Suari, 28, and his uncle, Jumadi, 48. Their extended family owns the strip of land where visitors can dock and hike over a wooden walkway to the lake.

On some days, hundreds of tourists arrive. But after all the visitors leave, life on Kakaban is lonely.

“It is really quiet here,” Mr. Suari said. “It is only the two of us.”

About 4,000 people, mostly Muslim , live on nearby Maratua , the largest of the Derawan islands. Most are Bajau people, renowned as deep sea divers, whose ancestors arrived here from the Philippines eight generations ago.

Darmansyah, a former chief of Bohesilian village on Maratua, said residents of the atoll were still mainly fishermen.

“Bajau people are not interested in farming,” he said. “We always run to the sea.”

But he is no longer fishing for a living, he said. Instead, like most other residents, he is happy to see a growing investment in tourism , including the recent construction of an airport and several new dive resorts