This portal tomb is an exquisitely preserved prehistoric site, made all the more arresting by the alien Burren landscape in which it sits. Its dolmen (or stone table) is huge, and surrounded by a natural pavement of rocks. The tomb has been dated back 5,000 years. When it was excavated in the 1980s, the remains of 16 people were found. And yet, it's still a mystery how the gigantic boulders were moved and lifted—the capstone alone weighs 4 1/2 tonnes (5 tons). In the summer, you probably won’t have much trouble finding this sight—just look for all the tour buses. At times they literally block the road.