The Thrihnukagigur volcano is dormant. It last erupted over 4,000 years ago. There are no indications of it erupting again in the near future. The volcano’s name, quite unpronounceable for anyone other than locals, would be directly translated as "Three Peaks Crater".
The three craters (one of which you will be descending into) are prominent landmarks, standing against the sky on the highland edge, about 20 km (13 miles) southeast of the capital area, and within the protected area of Blafjoll Country Park.
The most north-easterly of the three peaks is a small cinder cone, standing about 35 m/100 ft higher than its surroundings. At the top of this cone is a funnel-shaped opening, about 4×4 m (12×12 ft) wide, this is the entrance of a huge 120 m (400 ft) deep, bottle-shaped volcanic vault, measuring 50×70 m (160×220 ft) at the bottom. Volcanic passages continue down to the southwest, to a total depth of about 200 m (700 ft).
The beauty of the crater mostly consists of the various coloration found inside and its enormous and to some extent intimidating size. To put it in context, the ground space is equivalent to almost three full-sized basketball courts sited next to each other and the height is such that the full sized Statue of Liberty would easily fit into the chamber. So make no mistake – this chamber is colossal!