A Welsh teacher dubbed "the extreme frogman" has become the first person to explore more than 500m of Canada's deepest cave. Dr Martin Groves, who teaches maths at Llanishen High School in Cardiff, has returned from a Pounds 10,000 expedition during which he braved freezing temperatures in the depths of Castleguard cave in Alberta. With six others, he skied 15 miles up a glacier to reach the cave. Carrying 70kg of equipment, he reportedly only turned back when he ran out of rope. He said he felt apprehensive beforehand, "because it had been a year in the planning, and a lot of people had dedicated time and money. The real satisfaction came when I was mapping what I'd found. It was very demanding, but we pulled it off," he told a local newspaper. He plans to return next year to investigate more of Castleguard's 25km of tunnels.