In most cases, when a volcano erupts, the magma chamber is filled with lava which subsequently cools and hardens, blocking the entrance. But at Thrihnukagigur, it is believed the lava solidified through the walls or simply drained back to the depth of the earth. The magma chamber remained exposed and intact, making this a completely unique phenomenon to Thrihnukagigur.
The volcano last erupted some 4,000 years ago, leaving an opening of approximately 4×4 meters in diameter, leading to a bottle-shaped volcanic vault. The magma chamber is about 120m deep and measures 50x70m at the bottom. An Icelandic tourist operator now provides curious adventurists a chance to explore this ancient volcano inside out.