Maeshowe

The Orkneys have a piece of prehistoric engineering every bit as impressive as Stonehenge. A 7.3-meter-high (24-foot-high) burial mound, dating to about 2800 B.C.E., covers a chambered cairn constructed with gigantic stones. When Vikings arrived in the 12th century they left their mark in the form of graffiti—celebrating looting—carved into the stone. During the shortest days of the year, the sun shines down the entrance to illuminate the rear wall of the main chamber with mind-blowing precision.