Þríhnúkagígur is a dormant volcano in west Iceland, famous for its enormous magma chamber. It is the only magma chamber in the world that can be entered, and has a depth of 213 metres (699 feet).
Explore the depths of Iceland on a lava cave tour.
Þríhnúkagígur was a normal volcano in Iceland’s Blue Mountain range, also call Bláfjöll. However, its last eruption, almost 2,000 years ago, had some very unusual consequences.
After an eruption, the magma chamber of a volcano, now free of pressure, usually remains full of lava that cools into solid rock until the next eruption. If the magma chamber is fully emptied, the volcano will usually collapse in on itself.
To the bewilderment of scientists, however, neither of these things happened to Þríhnúkagígur.
It seems that a rift beneath the chamber drained it of all remaining lava, leaving behind an enormous chamber, that covered an area of 3,270 square metres (35,200 square feet). To give some idea of the scale of this, the Statue of Liberty could stretch her limbs within it, and it could comfortably fit Hallgrímskirkja church.
Unusually, the volcano did not collapse, and it has remained unchanged since.
The vast magma chamber under Þríhnúkagígur would not be discovered until 1974, by an Icelandic cave explorer. Its discovery made world news, as volcanologists had not seen anything like it before, and were fighting to try to explain its existence.
In 2012, it was opened for tourism. A lift, akin to those used in mining, was installed, and visitors were now allowed to join tours plumbing its depths.
Its popularity comes not only from the uniqueness of the tour and the scale of the chamber, but the incredible colouration within it. Mineral deposits dye the walls vivid shades of red, yellow, green and blue, making the incredible space that much more ethereal.
Reaching Þríhnúkagígur requires a short but uphill hike, which can be done by any steady on their feet with a reasonable level of fitness. Tours only operate in summer.