Known as Raufarhólshellir, this 11th-century lava tube is 1360m long (Iceland's third largest) and contains wonderful lava columns. Regular one-hour tours run on the hour every hour. The thrilling "extreme" tour guides visitors deep into the cave, where they have to climb over boulders and squeeze though tight spaces. Helmets and flashlights are provided but bring sturdy boots.
Tours run year-round. Extreme tours start at 11am every day and last three to four hours. In winter cold air is funneled down and trapped inside, producing amazing ice formations; bring warm clothes.
