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 (adult/child 19,500/9950kr) 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 funnelled down and trapped inside, producing amazing ice formations; bring warm clothes.