THERES A MOMENT IN THE FILM Close Encounters Of The Third Kind when a flying saucer comes overhead, and the people below are plunged into darkness by its shadow.
I had the same experience while diving in St Abbs - with a massive shoal of herring!
The summer sun was creating beautiful shafts of yellow and green in the plankton-rich water, and I know that such light effects always look good on video, so I descended slowly, trying to get the best angle on the suns halo (I was using a Gates housing for a Sony Z1 camera).
The seabed was only 20m below and I drifted down, not realising that I was about to have one of the most memorable experiences of my diving career. Suddenly it all went dark; I dont mean slightly grey, but pitch black. Who turned the lights out Whats going on
A shower of silver flakes transfixed me, flickering as they tumbled in the beam of my video lights.
I was really disorientated. It had been mid-day when I had dived, 10 minutes before, and now it was night.
I looked up to see a whirling tornado of tens of thousands of fish silhouetted against the surface. Now the sudden darkness started to make sense.
I had tagged along with members of the Bristol Underwater Photography Group at the St Abbs Splash-In, the now traditional underwater photographic competition at the end of August.
I think video is seen as some kind of cheating by stills photographers, but I explained that I was just taking 25 stills a second, and everyone was very friendly.
On the way to the dive, 15 minutes north of St Abbs harbour, we had been excited to see several minke whales about half a mile off shore.
We had heard that they had been there all day and, thinking about it, that means that they were on to something good: massive shoals of herring that they had perhaps chased inshore, and trapped in the relatively shallow water.
The whales were having a feast, and what we were diving under was one of the massive shoals trying to escape.
I soon realised that if I was going to see the herring close up, I had to keep still and stop breathing so much. The slightest noise made them panic and change direction, and disappear into the dark leaving a glittering trail of their loosely attached silver scales.
You cant really blame them. After all, they were being pursued by one of the oceans largest and most efficient predators.
But when I calmed down, I did get close to silver walls of spinning fish, from seabed to surface. They went past in a glistening blur with a motion that made you dizzy, parting and reforming like flocks of underwater starlings.
I tried to be still, and they were moving, but at times, as the conveyor-belt of fish sped by, it seemed to be the other way around.
I hoped the herring would part and reveal a minke-whale mother and calf coming straight to camera, but I knew the chances were similar to getting a shot of Elvis surfing on the nose of the Loch Ness monster. Maybe next year!

THIS MASSIVE SCHOOL OF FISH was a fantastic sight in UK waters, and I was reminded that our green seas are potentially among the most productive in the world.
Despite everything, here in St Abbs was a shoal that could by measured only in tons. I have been lucky enough to film in the open ocean from Colombia to Australia, but diving under those herring was one of the most spectacular moments Ive ever had. Truly a close encounter of the best kind.

Darkness closes in for the divers at St Abbs.
John Ruthven
* Can weigh up to 10 tonnes
* Are the favourite food of killer whales
* Can swim at more than 12mph
* Were named after an infamous rule-breaking 18th-century Norwegian whaler
* Have a very mechanical song that caused problems for submarine-hunters in WW2