Surtshedlir Cave

Surtshellir runs for about a mile beneath the Hallmundarhraun lava field, in the highlands above Borgarfjörður in west Iceland. For centuries it was the longest lava cave known anywhere. Named after Surtr, the fire giant of Norse myth, it is a raw, unlit tube of collapsed basalt. Its westernmost part, the 'ice cave', is floored in ice all year, and excavations near the mouth found animal bones from Viking-age rituals. No lights, no railings — come well equipped.