More than thirty islands break the surface of Nong Han, and the largest of them, Don Sawan, carries the ruins of old Buddhist structures — enough that the island has archaeological weight as well as a good name, since don sawan means something close to heavenly mound. English sources have sometimes rendered it as Heavenly Hill Island, which is the same idea wearing a stiffer collar. The drowned-city legend does not belong to Nong Han alone. It is told just as insistently about Nong Han Kumphawapi, another lake southwest of here in Udon Thani province, and the two claims have coexisted for as long as anyone has bothered to write them down.

There is a version of the story that geologists tell, and it is not entirely different. The Khorat Plateau lies over thick beds of rock salt. Groundwater dissolves the salt, cavities open underground, and eventually the ground above them subsides — collapse basins forming slowly over a very long time, which is a reasonable description of what a naga army would leave behind. Nong Han covers roughly 125 square kilometres, or about 77,000 rai, making it the largest natural lake in northeastern Thailand and by most reckonings the second largest in the country. It is also astonishingly shallow. The average depth is around two metres. Only the deepest holes reach ten, and in the dry season the margins simply become land.

The Nam Pung comes down off the Phu Phan mountains to the south and fills the lake. The Huai Nam Khan drains it to the southeast, eventually delivering the water to the Mekong and, a long way further on, the South China Sea. Between those two points the lake spreads across a dozen subdistricts, and its shoreline is anything but wild — Sakon Nakhon town stands on the southwest bank, much of the near shore has been laid out as public park, and Sakon Nakhon Hospital sits directly on the water. On a hot evening the lakeside fills with people walking, eating and watching the light go orange over the shallows.

Roughly 240,000 people in some 80,000 households across thirty-two subdistricts depend on Nong Han in one way or another — for irrigation, for household water, for fish. About a fifth of the catch is Puntius brevis, a small silver barb that ends up grilled, fermented, pounded into paste and folded into more or less everything. Waterfowl breed in the shallows. Rice paddies press up to the edge on every side. For a lake with an average depth you could stand up in, Nong Han carries an extraordinary amount of a province on its back. The pressure shows. It contracts hard in the dry months, spreads back across low ground in the wet ones, and holding those two facts in balance is the permanent work of everyone who lives around it.

Do not swim in Nong Han. The warning is standard in local guidance, and the reason is a flatworm called Opisthorchis viverrini, the Southeast Asian liver fluke, which cycles between snails, freshwater cyprinid fish and people. Infection comes overwhelmingly from eating the fish raw — koi pla, larb pla, the fermented preparations that are genuinely beloved here and are also, in this one respect, dangerous. The fluke settles in the bile ducts and stays for decades. In 2009 the International Agency for Research on Cancer classified it a group 1 carcinogen, definitively cancer-causing in humans, and Thailand now records the highest incidence of the resulting bile-duct cancer of any country on earth. Mass treatment campaigns have run since 1994 and infection rates have fallen. But the arithmetic is cruel: the cancers appearing now were seeded thirty and forty years ago, in meals eaten by people who had no way of knowing. The hospital on the lakeshore treats them.