Thrihnukagigur

Þríhnúkagígur (‘Three Peaks Crater’) is a dormant volcano on the Reykjanes volcanic belt, about 30 km south of Reykjavík, that offers the most extraordinary underground experience in Iceland — and arguably on Earth. Unlike most volcanoes where magma hardens in the vent, the magma here drained back down when the eruption ended roughly 4,000 years ago, leaving behind a hollow magma chamber the size of a six-storey building. Visitors descend 120 metres in an open elevator cage lowered into the chamber, where the walls glow with streaks of orange, red, purple, and blue mineral deposits.

The experience is operated exclusively by Inside the Volcano tours and runs from late April to late October. After a scenic 45-minute walk across the Bláfjöll lava plateau from the base camp, a guide lowers the group into the chamber in a small open cage, spending 20–30 minutes exploring the extraordinary interior before ascending. Total tour time is approximately five to six hours including the hike and a snack break at the base hut.

The tour is strenuous enough that reasonable fitness is required; the hike involves uneven lava terrain. Children under eight are not permitted inside the chamber. Warm, windproof clothing is essential on the plateau even in summer. Booking well in advance is strongly recommended — tours are limited by the size of the descent cage and sell out weeks ahead in peak season. The base camp is accessible by a short drive from Reykjavík followed by a walk from the car park.