Seljavallalaug is a 25-metre outdoor pool set in a narrow valley about 10 kilometres west of Skógar, at the end of a 15-minute walk up a river gorge. It was built in 1923 in a hollow beneath a cliff, where a natural hot spring provides the water, and it looks exactly as old as it is — the changing facilities are basic wooden huts and the pool is lined with algae and has no filtration system. It is entirely wild, uncurated, and genuinely beautiful.
The pool is not lifeguarded and technically has no opening hours. There is a small car park at the end of a rough track off the Ring Road, and then a 20-minute walk up the valley. The pool was partially buried by ash during the 2010 Eyjafjallajökull eruption and was cleaned and reopened by volunteers. In summer the area is busy; in the shoulder seasons it is often deserted. The water temperature varies around 30-35°C — warm enough to be comfortable, cool enough that you notice the outdoor air. Wear old clothes or a swimsuit you don't mind getting discoloured.