The Long Street Green Light (#UFOSA), Cape Town

Late November 2015. Cape Town was weeks away from the first HSBC Cape Town Sevens, set for 12 and 13 December at Cape Town Stadium. The city was preparing for visitors. Nobody expected them from quite so far away.

Saturday 28 November 2015, night over the City Bowl. South African Twitter began to fill with photographs of a green light. It did not seem to move. People described it as hanging in the sky over Cape Town. Many of the posts carried the same hashtag, #UFOSA.

The best-known image shows the light above Long Street. Nick Frost, posting as @Nick_Frost, wrote: "Everyone's tweeting about a UFO sighting off Long Street. Please let it be aliens. But nice aliens. #UFOSA"

Not everyone was charmed. Marco Rosa suggested that "some guy probably put a green light onto his new drone and spawned a hashtag #UFOSA". Hans Haupt complained there was "not one single video." The next day Tom Eaton pictured the "Marketing guys behind #UFOSA watching tweets anxiously."

Sunday night, 29 November. People claimed to see the light over Durban. A photo of a light above Moses Mabhida Stadium later turned up, wrongly, in a YouTube video that called the lights "inter-dimensional portals." Then the story moved north to Johannesburg.

Tuesday 1 December. A crashed spaceship appeared beside the road in Sandton, near Republic Road and William Nicol Drive. Officials in hazmat suits stood among the wreckage. Chief Supt Wayne Minnaar of the Johannesburg Metro Police put it plainly: "The advert storyline involves a spaceship and that has got the entire city in a frenzy." He added: "Traffic has been backed up from Republic Road moving towards William Nicol Drive."

In Cape Town, the South African Astronomical Observatory told News24 it understood the lights were a test of light-show equipment at Cape Town Stadium, based on something heard on the radio that morning. The stadium referred questions to the rugby union, SARU. Its spokesperson, Marlon Kruger, was unconvinced: "If it was a test, I would have known about it." "It is probably some sort of marketing campaign." He called a tweet from the official HSBC Cape Town Sevens account, which reported a UFO and invited readers to "Meet the Welcoming Committee", ambush marketing.

On 2 December Clover SA and FutureLife confirmed it. The sighting was a marketing stunt to launch FutureLife Smart Drink, sold as "the drink from the future." The campaign was conceived by Joe Public United, with PR by Engage Joe Public. The social agency emote seeded the UFO videos and pictures on Twitter and Facebook. The production company The Front built and filmed the crash site.

"UFOs always bring out the skeptics," said Adam Weber, Executive Creative Director at Joe Public. Ryan Silberman, CEO of emote, spoke of "over 15 million social impressions." #UFOSA became South Africa's number one trending hashtag. A later Cannes Lions entry for the campaign described footage and photos fed to newsrooms under false names, though those claims remain unconfirmed.

One question was never answered. No source says how the green light over Long Street was made. A drone, a laser, a lit balloon, or only doctored images. The case lives on as an advertising case study, and as a lesson in how fast a single light in the sky can become a national sighting.