In the last week or two, an astonishing video clip showing lightning reticulating around the base of a volcanic ash cloud has been going viral on Facebook, Twitter, and elsewhere. Taken from the first episode of the new series Patagonia: Earth’s Secret Paradise, it is awe-inspiring, and a superb achievement for the series.

You will probably then be disappointed to learn that you have been conned: what we saw was not what actually happened, but the product of digital compositing. It was a deception, not documentary, but imaginary. It was actually not even physically possible.

Tuppence Stone explains on a BBC blog how the sequence was made from a composite of two different video clips: one of Calbuco’s eruption this year, shot as a timelapse, and superimposed shots of lightning from a different volcano, Cordón Caulle, from four years ago, using long-exposure shots to capture the lightning.

Stone claims: “While these events happen naturally, they can be difficult to see with the naked eye or be captured on a single camera.”

That reads like an amazing circumlocution for “You can’t see them”, and it is, as what we were shown is not physically possible, and does not occur. Given that ash clouds do not billow as quickly, each frame in the ash cloud clip must represent at least a minute of real elapsed time. Yet the lightning, which in reality is only visible for fractions of a second, appears to have occurred over several minutes, judging by the number of frames in which it occurs.

This is not “difficult to see” but actually not a representation of the real world. It is imaginary, and thus has no place in documentary filming.

The BBC needs to act decisively over this piece of deception, which discredits the efforts that so many other documentary film-makers have gone to, to document what really occurs. At a time when the BBC is fighting to retain its traditional model of funding, and the scale of that funding, it needs to defend its standards.

If its new standards include releasing spectacular but imaginary footage to promote its programmes virally, then it will find few prepared to defend its dependence on public money.

Thanks to @adambanksdotcom and @mediaguardian for drawing attention to this story.