Seljavallalaug Zwembad is one of the more underrated swimming spots in southern Iceland. While the water is naturally heated from geothermal sources, a human-made pool encloses this water, and bathrooms are provided for people to change into bathing suits.
The pool is about a 15-minute hike from a parking lot that is easily accessible through a somewhat bumpy, dirt road. The quick hike to the pool borders a freezing cold river made from glacial melt, but as you walk to the pool, streams of steaming hot water come down the mountain and make their way into the river.
The structure of the pool looks like a normal swimming pool, and the water is dark green and deep at one end and shallow on the other. There is a ladder where tourists can enter and exit the pool, and there are tubes that release hot water into the pool, though the majority of it is lukewarm.
The pool resides on the side of a mountain in a valley, and it is surrounded by luscious, green mountains on almost all sides that enclose it. The coolest feature of the pool is the process of how it is heated and the contrast with the cold air and freezing river.
The pool was originally built in 1923 to provide a pool where locals could learn how to swim, since many of them had to pass a swimming test to graduate school.
The best season to go to Seljavallalaug Zwembad is during the summer when the air temperatures are cool and the water warms you. But luckily the hike to the pool is just short enough that you don't get cold in your wet bathing suit on the way back. However, in the winter and other seasons, the hike and experience would be much colder and not as pleasant.