The Fort Augustus Wave Video, Loch Ness

Fort Augustus sits at the south-western end of Loch Ness. Here the Caledonian Canal and the River Oich meet the loch, beside Inchnacardoch Bay. The pier head looks out over open water and the boat traffic that passes through the canal locks.

Late August 2013. David Elder, 50, a photography enthusiast from East Kilbride in South Lanarkshire, was at the pier head photographing a swan. "We were at the pier head," he said. Someone was with him. Their name has never been reported. The water was very still. "There was nobody on the loch at the time, no boats, nothing."

Then, out of the corner of his right eye, he saw it. A black area of water about 15ft long. It developed into a kind of bow wave. "The disturbance in the water began moving up the Loch sideways," Elder said. He filmed it.

"It looks like the sort of wave perhaps created by a windsurfing board," he said. There was no board. No head, neck or body broke the surface. Yet Elder was "convinced this was caused by a solid black object under the water." "Water was definitely going over something solid and making the wave," he said. "It is something I just can't explain."

The video, said to run about five minutes, and a set of stills appeared in the Daily Mail from 26 August 2013. The Mirror, The Scotsman, Metro, HuffPost and IBTimes followed, and the story travelled as far as Brazil. The Scotsman's headline read "New photo of Loch Ness Monster sparks debate".

On the Loch Ness Mystery blog, Roland Watson, writing as Glasgow Boy, studied the frames. The dark wave runs across calm water from right to left, then slowly dies away. He found no boat up the loch in shot and could not see the second arm a boat's bow wave would leave. He left it open: "A freak wave as someone suggested or something just below the surface disturbing the water?"

Sceptics answered quickly. Long-time researcher Dick Raynor wrote that "There is no need for anything to 'be causing' the waves, just something to 'have caused' them." His verdict: "None are required, just boats." Daniel Loxton of Skepticblog noted that everyone agreed the footage "shows a wave". He argued that boat wakes reflect off the loch's steep shores, cross and amplify one another, and build odd patterns long known to Nessie hunters.

Others suggested a freak gust of wind, a tree log, or the inflow of the River Oich meeting the sluices of the Caledonian Canal locks. In 2016 Martin Curran said it "Looks like a solid dark object, just under the surface", while conceding that wave interference could produce the same effect.

Gary Campbell, president of the Official Loch Ness Monster Fan Club and keeper of its sightings register, reviewed the year's most prominent reports. Two images, he said, "appear to be waves or the wake from a boat". The third was a duck. None was registered, and Elder's video is not on the register. 2013 became the first year since 1925 with no registered sighting. No one has ever suggested Elder staged his footage. It remains one of several cases from that season of a wake with no boat, alongside the Apple Maps image credited to Peter Thain.