By John Balson @JJBalson
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The armada, which is believed to be the biggest gathering of sperm whales in living memory, was photographed 25 miles off shore by British photographer and filmmaker Andrew Sutton.
The 54-year-old was given special permission to enter the water with the enormous animals, which can weigh more than 50 tonnes.
He said: "Our experienced estimates gave us a figure of around 350 individuals, though this figure was most likely higher toward 500.
"All the time I was being sonically scanned and the sensation was like gentle itching powder and my ears rang.
On the surface the sound was very audible to those in the boat.
"It's always good to have fear in these situations though you need to be confident and calm when faced with this number of powerful animals.”
Andrew had been on a trip with environmental group Whale and Dolphin Conservation (WDC) and a Sri Lankan marine conservation biologist off the north-west coast of the island on Tuesday, March 24.
Andrew, from London, said: "We came across the first group on the surface who were heading south.
An initial count gave us a figure of around 10-15 animals.
I made my first in-water approach, but they were heading off at some speed, though I could see at least another 20 or so animals deep underwater, which weren’t visible on the surface.
"When we stopped to decide our next move, we could see behind us to the north, a staggering amount of breaching in the distance, and in the foreground were more whales spread as wide as our eyes could see.
"Glancing back at the lead group, we became aware of more animals, all around us.
"My colleague Rob Lott from WDC was noting other groups about 100-150 metres of either side of us with even more further beyond.
We estimated this wave at about four groups of 15-20 on the surface and 20-25 underwater.
"Once again, I sheltered by the boat to see an even greater number of animals from the surface all the way down to the deep. They were almost indistinguishable in the dark depths.
"I observed many female whales upside down, presenting white flashes on their bellies - ready to mate most likely.
"In the middle were single huge males, the size of which I’d never seen.
Their massive heads looked something like the classic American trains of the 1940s, which made my heart pound.
"Sperm whales were surrounding us in a total 360 degree view as far as our eyes could see.
"A number of less intense, smaller groups passed us, which appeared to be nursemaiding groups.
I was back in the water and to my right came a small juvenile of about 15-20 feet in length, who spy-hopped me head on, then started approaching me.
"He came within 10 feet of me head-to-head and we stared at each other for about seven minutes.
All the time I was aware of six much larger animals, about 40 feet below me, all upside down, in a ring watching and sounding me out.
"I began to feel examined. I slowly retreated, and as I did, one of the larger animals moved in and seemed to nudge the juvenile in my direction, prolonging the encounter.
"Not bad for a day's casual whale-watching.”
Afterwards Andrew and the WDC team contacted world's leading sperm whale expert, advisor and friend of WDC, Professor Hal Whitehead of Dalhousie University, Halifax, Canada.
He said huge congregations like these have been confirmed in the past - most significantly a report from 1945 where an estimated 1,000 were seen in the Aguja Cape, Peru.
Sperm whales are the largest of the toothed whales.
They can grow to 67ft long and have the largest brain of any animal known to have existed.
Like many whales, sperm whales are currently listed as a vulnerable species by the International Union for Conservation of Nature.