A skipper has claimed a submarine may have snagged itself on his trawler as it fished off the Outer Hebrides.
Angus Macleod said he and his four crew were "extremely lucky" after his net was continually dragged in front of his 62ft boat.
The Royal Navy has said there were no British or Nato submarines in the area at the time.
There has been speculation in recent months that Russian subs have been operating off the Scottish coast.
Mr Macleod's wooden Aquarius boat was fishing for haddock, monkfish and skate about 10 miles east of the Butt of Lewis in 360ft of water on Tuesday evening.
The boat had two nets out when the port net suddenly moved in front of the boat, while the other continued to lay astern.
Mr Macleod, 46, said he was baffled by what was happening and had to "up the revs" on the engine to try to keep ahead of the net for fear of it being entangled on the propeller.
He said: "It kept going forward and we had to repeat the manoeuvre four times to stay ahead.
"The winch became increasingly under strain as we tried to haul the rope. There was no way the net was snagged on the bottom - we were fishing well off the bottom. It only ended when the dog rope, which attaches the top and bottom ends of the net, was cut by the propeller.
"I have been at sea for 30 years - and between the five of us there is 110 years experience - and in our collective times we have never experienced anything like that.
"The sea conditions were good. We were mystified - we just couldn't explain it."
Mr Macleod, from Barra, believes his boat suffered about £10,000 of damage in the incident, with his trawler having to be towed back to port by the Stornoway lifeboat when its steering developed a fault.
He said he had lodged an incident report with the Maritime and Coastguard Agency and the Marine Accident Investigation Branch.
He asked: "Is it possible a non-Nato submarine could be involved? It was not a whale - we have had whales in the nets before and the net is all twisted afterwards. Whatever it was was human powered, of that we are convinced.
"I think that something got hold of the dog rope and the trawl wire. The only explanation I can think of is it's a submarine. It missed the nets, which is just as well. All five of us are extremely lucky. I don't even want to think of the consequences of what could have happened.
"I would like to get to the bottom of it. It was a very worrying experience - it shook us all up especially when we thought about it afterwards. Whatever happened involved a moving object in mid-stream."
A Royal Navy spokesman told BBC Scotland that there were no UK or Nato submarines in the area at the time.
He declined to speculate on whether a foreign submarine may have caused the incident, but said the Navy always took seriously and tried to investigate allegations that foreign submarines may be in British waters.
Four fishermen died in the Firth of Clyde in 1990 when the Scottish trawler Antares was dragged under by the nuclear-powered HMS Trenchant submarine.
In November, the UK called on the help of aircraft from Nato allies after a reported sighting of a submarine periscope off the west of Scotland.
The search continued for several weeks, with planes from the US, France and Canada flying out of RAF Lossiemouth.
The MoD would not confirm it was looking for a foreign submarine, but there has been an increase of Russian military activity in recent months.
Nato and the Royal Navy are shortly due to stage Joint Warrior - one of Europe's largest military training exercises - off the north of Scotland.