In hot water: swimming Iceland's volcanically heated pools

Bathing outside in naturally heated water is part of Icelandic life. Icelanders are expert at capturing hot water, either in hillside hot pots (little pools sunk into the ground, made from anything - stones, concrete, repurposed agricultural tubs) or outdoor pool complexes with water slides.

Our holiday mission is simple: to travel around the country hunting down volcanically heated water. The materials for our expedition are considered and gathered as if we are going on a mountaineering trip: base layers, dry bags, thermal jackets and Gore-Tex.

The trip begins at the hot spring area at Hveragerdi, in Iceland's milder, easier south, 45km from the capital, Reykjavik. It's a land of thundering waterfalls and steaming hills, where water spurts and forms pools. The hills are patchy with grass and bare iron-red earth. A pony drinks from a hot stream and we take our first dip.

Two of the south's best pools are also among the country's oldest. At the Secret Lagoon, in Flúdir, built in 1891, the changing rooms are swish modern Scandi but the pool edges are raw and grassy. A small geyser nearby blows with regularity and pots of bubbling mud are partly obscured by steam.

Seljavallalaug, which dates from 1923, is more basic. Set in a valley and backed by a cliff, with a snow-capped glacier beyond, it occupies one of the most stunning swimming pool positions on Earth. The water, at a family-friendly 36 degrees Celsius, is a deeper green than any I've been in before. We emerge two hours later with wrinkly fingers while a more recent arrival performs somersaults to assembled applause.