Ancient Scandinavians dragged 59 boulders to a seaside cliff near what is now the Swedish fishing village of Kåseberga. They carefully arranged the massive stones — each weighing up to 4,000 pounds (1,800 kilograms) — in the outline of a 220-foot-long (67-meter) ship overlooking the Baltic Sea.

Archaeologists generally agree this megalithic structure, known as Ales Stenar ("Ale's Stones"), was assembled about 1,000 years ago, near the end of  the Iron Age, as a burial monument. But a team of researchers now argues it's really 2,500 years old, dating from the Scandinavian Bronze Age, and was built as an astronomical calendar with the same underlying geometry as England's Stonehenge.

"We can now say Stonehenge has a younger sister, but she's so much more beautiful," said Nils-Axel Mörner, a retired geologist from Stockholm University who co-authored the paper on the interpretation, published in March in the International Journal of Astronomy and Astrophysics.