Dimmuborgir or “dark castles” is an area of lava formations that have several different walks you can take around well-marked paths. It a strangely quiet place given how enclosed it actually is, and that makes it a little spooky. It’s very easy to get away from other tourists here as most of them seem to do only the shortest of the walks. Here are some oddly shaped formations, including the famous “church” entrance.
I have posted on a previous trip to this same place here.