Seljavallalaug is a geothermal pool that was constructed in 1923 to provide locals with a place where they could learn to swim. It also happens to be the oldest pool in Iceland that is still standing.
For years, Seljavallalaug was a secret spot known only by the locals. Hence, it's nickname being "The Secret Pool". Recently, it's gained more traction with tourists and travelers alike so it's not quite a secret anymore. Still, it's less crowded than most of Iceland's bucket list attractions because it is a bit more difficult to get to.
Located in a quiet valley surrounded by mountains, Seljavallalaug is completely invisible from the road.
To reach the pool you'll walk for around 15-20 minutes into the valley, following the uneven paths and jumping over a stream. If you don't see it right away, that's normal; you can't see it until you get to it.
If you're expecting a chlorinated pool like you'd find at your friend's house or your swim clubs, don't. It's more like swimming in a river, or a lake, than what we naturally think of as a swimming pool.
The pool is filled with hot spring water that trickles down from Eyjafjallajökull, a volcano completely covered by an ice cap.
Still, the water is warm and the views are incredible so I think it's worth it.
All you have to do is lean against the edge of the pool, look at the mountains surrounding you, and you'll be left marvelling at the hidden gems Iceland has to offer.