Rory Nugent's Mokele-mbembe sighting and photographs at Lake Tele

Mokele-mbembe. A creature of the swamps and rivers of the Congo basin. To its hunters, perhaps a surviving dinosaur. By the mid-1980s Lake Tele, a shallow lake in the Likouala region of what was then the People's Republic of the Congo, had drawn expedition after expedition. The Powell and Mackal expeditions. Herman Regusters. Marcellin Agnagna.

Rory Nugent was an American travel writer and self-described amateur cryptozoologist. He travelled alone and without sponsors. In Brazzaville he waited about 40 days for permits. He also paid a witch doctor, who stripped him, smeared him with a foul-smelling jelly and exorcised his "devils".

A twin-engine Cessna, then a pirogue to the village of Epena. Then on foot and by canoe through swamp forest to the lake. Along the way he met Pygmy hunters, and one group threatened him with bows and arrows. There were poachers, swamps, "skin borers" and venomous snakes. At the lake he spent weeks exploring.

Then, from the shore, he saw it. About a kilometre away, in the middle of Lake Tele. An elongated black form that curves in on itself. A periscope-like shape, moving through the water. Accounts paraphrasing his book say it was shaped like a "slender french curve" and moved "with little wake".

Nugent took photographs. Then he tried to get closer. His guides stopped him at gunpoint and threatened to shoot him. Their reason was simple: "the god can approach man, but man never approaches the god."

Those pictures are the only physical evidence. Publishers Weekly reported in 1993 that experts who examined the photos in New York "were not sure what he captured". William Gibbons, whose Operation Congo expedition, sponsored by Fortean Times, was in the region at around the same time, reportedly said the image "could be anything". Karl Shuker reportedly found it too blurred to be of much value.

Even the date is soft. The sighting is conventionally placed in December 1985. One review of his book calls it his "1986 expedition".

In Abominable Science! (2013), Daniel Loxton and Prothero call the photographs unreliable. Prothero argues the object is "more likely a floating log". Others suggest a branch. At a kilometre, the image is too indistinct to settle the matter. No one has accused Nugent of a hoax.

Nugent told the story in Drums Along the Congo: On the Trail of Mokele-Mbembe, the Last Living Dinosaur, published by Houghton Mifflin in 1993. John Leonard reviewed it on NPR's Fresh Air. The publisher claims believers have reprinted the photos hundreds of times. What moved across Lake Tele that day is still argued over. A dark curve, far out on the water, that no one was allowed to approach.