Caving in the Reykjanes Peninsula

Þríhnúkagígur is the only place on earth where you can go inside a volcano's magma chamber.

An open lift cage lowers you 120 metres down the throat of a dormant volcano into a vast, cathedral-sized void, its walls streaked in reds, purples and golds where molten rock once churned — a space so large it could swallow a building, and one no other volcano on the planet lets you enter.

For a gentler taste of Iceland's volcanic underworld, the nearby Raufarhólshellir lava tube offers a walkable, lit passage through a tunnel that a river of lava carved and then drained.

Together they show the two faces of Icelandic caving: the once-in-a-lifetime descent into a magma chamber, and an accessible walk through the plumbing of an old eruption, both a short drive from Reykjavík.

Two very different outings. Þríhnúkagígur is a guided half-day: a hike across lava to the crater, then a five-minute ride down an open window-cleaner's lift into the magma chamber, where you stand on the floor of an extinct volcano surrounded by colossal coloured walls.

It needs no caving skill, just a head for the airy descent and reasonable mobility, and it runs only in the short summer season for a limited number of people.

Raufarhólshellir is easier still — a guided walk on paths and steps through a lit lava tube, cool and dark, suitable for most people and families. Both are cold underground, so warm layers, sturdy shoes and the supplied helmet and harness (for the chamber) are needed.

The scale and colour underground are the unforgettable part.

Þríhnúkagígur runs only in summer, roughly mid-May to October, with strictly limited daily places — book well ahead, as it sells out, and be ready for the 3 km hike each way plus the lift descent.

Raufarhólshellir runs year-round with standard and more adventurous 'lava tube' options and needs no advance skill. Both are guided; you can't visit the magma chamber independently. Wear warm layers (it's cold underground even in summer), sturdy closed shoes, and expect to don a helmet and, for the chamber, a harness.

Neither requires caving experience, making them suitable for beginners and, for Raufarhólshellir, families. If the magma-chamber descent is your goal, plan your trip around its short season and book early — it is genuinely unique.