Þríhnúkagígur, anglicized as Thrihnukagigur, literally translated as Three Peaks Crater) is a dormant volcano near Reykjavík. Covering a 3,270 square metres (35,200 sq ft) area and a depth of 213 meters (699 ft), it has not erupted in the past 4000 years.

It was discovered in 1974 by cave explorer Árni B Stefánsson, and opened for tourism in 2012. It is the only volcano in the world where visitors can take an elevator and safely descend into the magma chamber. The magma that would normally fill the chamber and become sealed is believed to have drained away, to the bewilderment of most scientists, revealing the rift beneath the surface. 

You can get a tour to see one of Iceland's most unique sightseeing experiences from companies such as  Inside the Volcano ,Gray Line and Extreme Iceland