Víðgelmir

Iceland's largest lava cave: 1,585 metres long with a main chamber up to 16 metres high and 18 metres wide. It formed from the same eruption that created the Hallmundarhraun lava field about 1,000 years ago. There's evidence of human habitation from the Settlement Age — bones and artefacts have been found inside.

The guided tour (about 6,500 ISK, 1.5 hours) follows a boardwalk with lighting that highlights the lava formations and mineral colours on the walls. In winter, ice sculptures form near the entrance. The cave stays at 1-2°C year-round — bring warm layers and sturdy shoes. The boardwalk makes it accessible to most fitness levels.

Víðgelmir is in the Borgarfjörður area of west Iceland, near Húsafell, about 2 hours' drive from Reykjavík. It pairs well with nearby Hraunfossar and Barnafoss waterfalls, the Langjökull ice tunnel, and Deildartunguhver hot spring. Tours run year-round and depart from the cave entrance. Book at thecave.is.