This is the opening of the main ice cave in Kverkfjöll, Iceland. The tunnel leads on into the massive Vatnajökull glacier for kilometers. All melted by this little stream of geothermal hot water. It is possible to wade up along the warm river far into the cave. However it is not recommended as it is very dangerous. The cave caves in every once in a while and large blocks of ice fall down from the ceiling. You may not realize this as they quickly melt in the hot water.
Lots more on Skarpi’s flickr.