Zhuo Yongsheng films six Lake Tianchi Monsters

Tianchi means Heaven Lake. It fills the crater of the Changbai volcano, known in Korea as Baekdu, on the border between Jilin Province, China, and Ryanggang Province, North Korea. The water is 373 metres deep. Hot springs keep parts of it free of ice in winter.

Stories of something living in it go back more than a century. In 1903, by one account, a buffalo-like creature attacked three people, was shot six times and retreated under the water. In August 1962 an observer with a telescope watched two creatures chasing each other. In July 2003 local officials reported white and black spots surfacing again and again. Days before September 2007, tourists said the monster was back, bigger than a person.

Thursday, 6 September 2007. Zhuo Yongsheng, a reporter for the television station of the Changbai Mountain Management Committee, was on the south slope at dawn, filming the sunrise. A tour guide stood with him. About 5:26 a.m. the guide spoke. There was an extra rock in the lake.

Zhuo pointed his camera at the centre of the water. One black object. Then five more came up from below. Six of them. They looked like seals, and each had a pair of fins or wings that drove it through the water. "Their fins, or maybe wings, were longer than their bodies," Zhuo said. "They could swim as fast as yachts and at times they would all disappear in the water."

The six moved together, sometimes in three parallel pairs and sometimes closer together, leaving circular ripples. For about an hour and a half they swam and frolicked. Zhuo filmed on and off for more than 20 minutes, until they were gone around 7:00 a.m. He had not believed the legend before, but now said, "I believed my own eyes."

Zhuo reported the sighting on 8 September and sent his stills to the Jilin bureau of Xinhua. A Chinese photo set ran on 10 September. Through AFP the story travelled from Beijing to the world, and papers called it China's own Loch Ness monster.

The doubts came quickly. Scientists cited by AFP said the cold water and volcanic activity made the lake unable to support such creatures. An eruption about 300 years ago, the argument ran, would have left it inhospitable. Sceptics also suggested floating volcanic rock, or imagination.

In November 2007 Kim Li-tae, a 77-year-old researcher at the Zoology Institute of the DPRK Academy of Sciences, gave a North Korean answer in Choson Sinbo. The monsters, he said, were probably trout stocked in the lake around 1960, a project he had taken part in. By 2000, measured fish reached 85 cm and 7.7 kg, and larger ones might live deeper down. A later Chinese retelling says an expert group decided the six were otters, swimming in unison and changing formation. That account names no group and gives no date.

No one has accused Zhuo of faking his film. But Tianchi's sightings sit inside a wider debate. In 2013 People's Daily asked whether monsters that appear every peak tourist season are real or hype. Students and monitoring staff have reported shapes and wakes since then, and The Mountain Goats sang about the creature in 2008. The question Zhuo raised that morning is still open. What were the six black shapes in the middle of Heaven Lake?