ÞRÍHNÚKAGÍGUR

The only magma chamber you can walk into

Þríhnúkagígur, which translates roughly as "Three Peaks Crater", is a dormant volcano in the Bláfjöll mountains a short drive southeast of Reykjavík. It last erupted around 4,000 years ago, and it holds a distinction shared by no other volcano on the planet: you can be lowered right down into its magma chamber. Most volcanoes seal themselves shut when they fall silent, the magma cooling and solidifying in place. At Þríhnúkagígur the molten rock instead drained back into the earth, leaving a colossal empty cavern behind.

How the chamber was formed

The chamber was unknown to the wider world until 1974, when Icelandic cave explorer Árni B. Stefánsson first descended into it on a rope. What he found was a space of extraordinary scale and colour. Iron, copper and other minerals in the rock have painted the walls in vivid reds, oranges, golds and deep purples that shift as the light moves across them, giving the cavern the feel of a natural cathedral carved out of fire.

Descending into the volcano

Visitors reach the bottom aboard an open cable lift, adapted from the kind used to clean tall buildings and cliff faces. The descent takes about six minutes, dropping roughly 120 metres (nearly 400 feet) through the volcano's narrow neck before the walls fall away and the chamber opens up around you. It is a slow, quiet journey that lets the sheer size of the space reveal itself gradually.

A cavern on a staggering scale

The chamber is enormous. Its floor covers roughly 3,270 square metres and it plunges to around 213 metres at its deepest point – large enough to swallow the Statue of Liberty with room to spare, or to hold Reykjavík's Hallgrímskirkja church standing upright inside it. Down at the bottom the air is cool and still, and the scale of the surrounding rock is difficult to take in until you are standing within it.

Where is Þríhnúkagígur and how to visit

Þríhnúkagígur lies in the Bláfjöll (Blue Mountains) area, only about a 20 to 30 minute drive from central Reykjavík, followed by a hike of roughly 3 kilometres across a lava field to reach the crater. The descent is only possible on a guided "Inside the Volcano" tour, and because of the exposed alpine setting these tours run only in the summer season, generally from May to October. The full excursion, including the hike out and back and time inside the chamber, takes around four to five hours, so comfortable hiking shoes and warm, weatherproof clothing are essential whatever the forecast.

For anyone fascinated by Iceland's volcanic landscape, Þríhnúkagígur offers a rare chance to see it quite literally from the inside – a once-in-a-lifetime descent into a place most people only ever imagine.