Iceland’s oldest swimming pool built in 1923 is located in a stunning mountainous setting within the Southern Region of the country. Seljavallalaug Pool is 80 feet long and 30 feet wide, it was the largest pool in the country until 1936. 
The facility was built to teach people how to swim, on an island where many careers dealt with fishing. Today, Icelanders cannot graduate school without knowing how to swim. Here tourists and locals alike can find a place to swim in the valley below the Eyjafjallajökull Glacier, free of charge.
How to get there:
To reach this site GoogleMaps GPS will not allow driving directions, even though there is a location in its database: Seljavallalaug Parking. However, it is fairly simple to get there by looking at your map, or with these simple directions. From the Ring Road turn onto Route 242/ Raufarfell. You will reach a junction labeled Seljavellir, turn onto this junction passing by the ‘Welcome Guesthouse Edinborg’ (which you can use as a GPS location to get this far). Take dirt road, manageable with any 2WD going slowly, till the fork and bare right. From here you can park in the first lot or tackle the road which deteriorates for a second slightly closer parking lot.
+If your GPS allows you to use coordinates simply enter: 63.5655° N, 19.6079° W
The Hike:
From the parking lot there is a one mile hike to reach the pool, so count on a two mile roundtrip adventure. The path is relatively flat and easy although it is not marked, just head up the valley sticking towards the left and you will reach the pool. 
There are a few streams to cross which were very shallow during my September visit. Don’t think of the hike as a chore to get to the pool, it enhances the adventure and is exceptional beautiful.
The good, the bad, and the ugly of Seljavallalaug Pool:
The Good: The scenery is beyond gorgeous; the hike is very fantastic and worth completing without even taking a dip. Visiting is entirely free.
The Bad: This is not exactly a hot pot, but more like a lukewarm bath. The water comes from a hot spring nearby and the temperature of the pools is between 68 and 86°F. If you stay near the pipe in the corner farthest from the changing room, closest to the path that is the warmest area. 
Also, the pool is cleaned once a year so there is often a healthy coating of algae, making it a bit skuzzy and slippery.
The Ugly: There are changing rooms, but they are in awful condition. Garbage and soiled clothing are piled up inside. Since this is a free place, it is up to visitors to maintain the site. It is such a shame to see a wonderful place ruined by disrespectful tourists, do not add to this debacle.
Pro-tips for natural hot springs:
+Wear hiking sandals (and socks to keep your toes cozy if you want to be really fashionable) which makes changing easier, without sacrificing sturdy footwear on your walk to the hot springs.
+Wear clothes that are easy to change in and out of and your bathing suit underneath them.
+Earmuffs or a beanie will keep your head warm while you soak in Iceland’s cold environment.
+Don’t forget to bring a towel!
+A plastic bag is great to throw your bathing suit in after changing.
Nearby: