Thórsmörk in South Iceland is a popular destination hidden within mountain ridges as well as Eyjafjallajökull volcano/glacier and Mýrdalsjökull glacier/volcano. The area lies within a system of volcanoes on the south coast of Iceland starting with the Westman Islands that suffered two major eruptions in the 20th century. A little further inland from here is the great Hekla known as the mother of all volcanoes and the gateway to hell in Medieval times. To the east is Mount Katla and absolute behemoth of a volcano erupting usually every 100 years or so and last erupted in 1918. Then there is the infamous Eyjafjallajokull volcano which is by no means among the most active volcanoes in Iceland erupting roughly every 200 years. The 2010 eruption of Eyjafjallajökull created an ash plume that stopped all air travel in the North Atlantic for a few days.
Thórsmörk is very much at the mercy of the elements, glacial rivers run through it and although Eyjafjallajökull is not a massive glacier then Mýrdalsjökull is the 3rd largest glacier in Iceland. The Black Sand Beaches to the south are a reminder that the volcanic eruptions in this area can easily reconstruct the landscape. The coastline has been moving back and forth for thousands of years in an endless tug-of-war between the new earth brought forth from by the eruptions and the gnawing away of the merciless north Atlantic. At Reynisfjara beach this is quite evident from the rock formations just off the coastline and the basalt columns rising out of the black sand.
There are quite a few popular hiking trails around Thórsmörk and you could easily spend days hiking here. Just off the ring road passing Seljalandsfoss waterfall the road gradually leads into the long Valley dominated by Krossá river and there is a surprisingly lush vegetation here in summer of birch, moss and fern which creates a stark contrast from the volcanic gravel and rugged mountain ridges.
You could easily combine a visit to Thorsmörk with a south shore tour if you are thinking about trying to get as much in as possible, visiting also Skógafoss waterfall and Seljalandsfoss waterfall as well seeing a bit of Black Sand for example down by Landeyjahöfn Harbour. The fact is though that a full day would be a recommendation for this area since it is rather unique. If you have already done the Golden Circle and some other highlights of Iceland this is very much a great option. Even if there are no ice caves in this particular area or glacier lagoons (there was one which was destroyed in the 2010 eruption) there are still plenty of amazing stops and just crossing the great river in a super jeep and or a monster truck is an experience in itself. There is also an accommodation option in the Volcano Huts in Thórsmörk, if that is something you would like to consider.
But all in all Thórsmörk is a little piece of heaven in the Lord-of-the-Ringy environment of the Icelandic highlands.