There are a number of unique caving experiences across Iceland but most pale in comparison to Búri, an immense, tubular cave created by irregularly cooled magma. 

Discovered fairly recently in 2005, the Búri lava tunnel is thought to be one of the most significant discoveries of its kind in the last 1,000 years due to its size and its one-of-a-kind formations. The tunnel runs over half a mile underground and at its largest point the cavern's size checks in at 32 feet tall and 32 feet wide. One of the most remarkable aspects of the long cave is that at some point it was full of rushing magma. The structure formed when the lava on the edges of the flow cooled much faster than the interior plasma, creating a solid tube while the excess lava drained out the end. There is still a 56-foot lava fall (read: hole) at the end of the tunnel marking where the lava drained out. 

In addition to the rocky tube itself, due to the chilly climate within the stone, huge, bulbous icicles form in both stalactite and stalagmite formations. Despite being forged in fire, it may be these ice formations that Búri is best known for.

Update: Unfortunately the cave is now closed and cannot be visited.