This 11th-century lava tube is 1360m long (Iceland's third largest), and contains wonderful lava columns. You’ll need a torch (flashlight) and sturdy boots, but even so the going is treacherous from earlier cave-ins. In winter, cold air is funnelled down and trapped inside, producing amazing ice formations. You’ll find the tube southwest of Hveragerði off Rte 39, which passes right over it – park on the south side. Local operators offer tours.
