CORK ultra-marathon runner Alex O’Shea has completed another extreme challenge by finishing fourth in a “treacherous” frozen lake marathon in Siberia.

The world-record holding firefighter was the only Irish runner in a field of 150 athletes at the Baikal Ice Marathon in southern Siberia — regarded as one of the most difficult marathons in the world.

The trip to Russia saw him take three different flights — to Frankfurt, Moscow and Irkutsk — a 1,600km round trip to get to the isolated marathon course but the travel did not affect his performance in the world’s only ice lake marathon race.

He crossed the line in a time of 3 hours 15 minutes and 58 seconds.

“I’m super happy with the time. I just couldn’t have imagined what this would be like.

“Light levels changed so much. A few miles in the visibility dropped and it was threatening to snow,” he said.

“I got to the halfway point in fourth place and it stayed that way for the rest of the race. There were two lads in sight but I was never going to pull them back.

“It was a hugely technical race. I was running on snow, through snow and on a black sheet of ice. There was kilometre upon kilometre of ice.

“It was absolutely amazing, I’d urge anyone to come out and experience it. The crowd was cheering everyone on the treacherous black ice as they were doing a sprint finish. It was a super, super experience culturally and athletically,” he added.

The surface of the frozen Lake Baikal is covered in fields of ‘hummocks’, which are small hills of ice rubble. Beneath the ice surface, geothermic springs cause localised melting that weakens the ice to form holes and cracks. Although it is mostly covered in a soft layer of snow, there are areas of highly polished ice that create conditions similar to an ice-rink. Strong winds add to the already bitingly cold temperature across the lake.

The course is regarded as one of the most psychologically and physically demanding in the world.

The Ballineen man completed a new feat in September when he became the first person to run 32 marathons, one in each county, over 16 days in aid of Irish Guide Dogs. In 2014, he broke the world record for running a marathon dressed in firefighter gear.