It becomes dark almost as soon as we begin our descent into Víðgelmir. It’s the kind of darkness to which your eyes will never adjust, and although above ground it’s a summery twelve degrees Celsius, the temperature has quickly dropped to near freezing and the air begins to taste stale. We’re marching deep into the largest lava tube in Iceland, a long and wide cave that stretches for nearly 1.6 kilometres beneath the Hallmundarhraun lava field.

More then just a cave

Since May of this year, the family who owns the land around Víðgelmir has been operating guided tours into the lava tube under the name “The Cave.” They’ve built a warm and brightly lit building that rises from the uneven grey rock near Langjökull glacier and acts as a starting point for the tours. From there it’s a 200-metre walk through the lava field to the gaping mouth of Víðgelmir, which was formed after a section of the lava tube collapsed.

“Let’s hope the cave doesn’t collapse today,” jokes our guide, Egill Örn Sigurpálsson, as we crouch through the narrowest part of the tube. Inside lies a cavern that has been fitted with lights to illuminate the green, reds and yellows that dress its walls. “This room is called ‘the cafeteria,’” Egill says, because it was where they kept the coffee whilst building the wooden ramps and stairs that guide visitors through the cave.

Egill explains how cold winter air is trapped in the cave year-round, keeping the patches of ice around us from melting. Icy water drips from the ceiling, forming tall stalagmites of ice that rise from the cave floor. Glittering in front of fitted lights, these crystalline formations shine like diamonds in the darkness. The rock walls themselves are coloured by rust, sulphur, iron and cobalt, as if painted by frenzied brushstrokes.

Reading history in the rock

A couple hundred metres deeper into the cave, Egill stops the group and jumps off the wooden platform. “This is my favourite part of the cave, because it reminds me of chocolate,” he says, resting his hand against the reddish-brown rock that appears to be melting off the walls. These unique geological formations were formed by lava from the eruption of a volcano beneath Langjökull glacier. As they cooled down, thin crusts formed on top of the lava, allowing different temperatures of lava to flow on top of each other and create visible layers on the cave walls.

Our journey ends in a cavern about 600 metres into the lava tube and 39 metres below ground level. At this depth, Egill’s earlier joke about the cave collapsing takes on a certain gravity. Boulders that dwarf the members of the tour are strewn around the cave floor—the remnants of other collapses that probably occurred during the lava tube’s formation. It’s only a matter of time before the whole structure collapses, through Egill assures us that won’t be for at least another 5,000 years.

Back above ground level, the air feels refreshing and warm. The Langjökull glacier, which had been shrouded by clouds when we began our descent, now peeks out from behind the neighbouring mountains. As we march back across the lava field towards the base camp I’m struck by what other worlds might lie just out of sight, only a few metres below our feet.