Leave from the central meeting point in Reykjavik and drive to the Raufarhólshellir Lava Tunnel, one of the largest and most popular in Iceland and only accessible on a guided tour.
Walk through the tunnel with your guide and learn about the volcanic processes that formed the tunnel thousands of years ago. This tunnel is of particular interest because of the beauty of its stalactites, whose many colors are illuminated by sunlight from the entrance.