The suspicion that the Discovery Channel had abandoned its professed editorial standards was a powerful one. As I mentioned in my earlier blog, its documentary claiming that the giant shark Carchardon megalodon still exists contained images which gave a strong impression of being faked; reports of incidents which don't appear to have happened; and interviews with "marine biologists" no one has been able to trace.

But allegations of fakery are very hard to prove. As you know, absence of evidence doesn't mean evidence of absence. Just because no one has been able to find the news reports the Megalodon show claims to have found, or any record of the deaths of four people in an attack by a giant shark off South Africa last year, or any trace of the suspiciously handsome experts it used to confirm its thesis doesn't prove definitively that all of them are inventions, even though it's hard to see how they could not be.

And pointing out that a photograph the "documentary" used to make its case looks like a really bad CGI cobblers in which just about everything is wrong isn't quite the same as being able to state categorically that it's a fraud.

Whale carcasss on beach. Photograph: theguardian.com

So to test my suspicions I offered a small reward – a signed copy of my latest book – to the first person who could find an original copy of another image Discovery used, which purported to show a Megalodon swimming past two U-boats off Cape Town.

It was the perfect cable channel conjunction: Nazi U-boats and a rediscovered extinct sea monster all in one frame. How clever they were to have found such an image, which, though utterly astounding, had remained unnoticed for 70 years!

Apart from the minor quibbles that no U-boats of this class are known to have been close to South Africa on the given date, that everything about the shark fins looks wrong, that at 64 feet between the dorsal fin and the tail this monster was twice the size even of the actual creature (which every expert on Earth, except the two mysterious "marine biologists" in the film, believe became extinct about 2 million years ago), and that the great beast creates neither bow wave nor wake, there were other reasons to be a little suspicious.

As one of my correspondents points out: "The swastika up the top is ludicrous so I won't bother mentioning that. The photograph is toned sepia. This is ridiculous as it required a separate pigment in a process that was used to make the photograph look warmer and 'nicer' for family photographs. It required more effort that developing in black and white. Photographs coming as sepia as standard is simply another myth created for entertainment."

So there's powerful evidence that this image had been doctored, but again it doesn't quite amount to proof. Until now.

Before I wrote the article I conducted an image search, and found nothing. Now I know why. It wasn't a still picture. A sharp-eyed reader found the frame in some footage of U-boats on Tarrif.net. The footage was shot in the Atlantic. Take a look 12 seconds in.

It's the same shot. But guess what? No shark. And no swastika. And not off Cape Town. Or anywhere near.

I wrote to the company handling media inquiries, putting it to them that the production company which made the film, Pilgrim Studios, doctored the image and misled the audience. I have not heard back from them.

Here's Discovery's mission statement:

Discovery Channel's mission statement. Photograph: Public domain

How many people now believe it's living up to these ideals?

Monbiot.com