Víðgelmir is a massive lava tube located in west Iceland, formed around 900 AD. It's the largest of its kind in Iceland, stretching 1585 meters long with openings near its north end. The cave has cool ice formations and evidence of Viking Age human habitation. Watch your step though, the cave floor can be rough! Guided tours are available at Fljótstunga for those brave enough to explore this natural wonder.