Ireland’s best-known prehistoric monument is one of the archaeological wonders of Europe. Built as a burial mound more than 5,000 years ago—long before the Egyptian Pyramids or Stonehenge—it sits atop a hill near the Boyne, massive and mysterious. Newgrange is so old, in fact, that when it was being built there were still woolly mammoths living in parts of Europe. The mound is 11m (36 ft.) tall and approximately 78m (256 ft.) in diameter. It consists of 200,000 tons of stone, a 6-ton capstone, and other stones weighing up to 16 tons each, many of which were hauled from as far away as County Wicklow and the Mountains of Mourne. Each stone fits perfectly in the overall pattern, and the result is a watertight structure, an amazing feat of engineering. The question remains, though: Why? Even as archaeologists found more elaborate carvings in the stones, they deduced no clues as to whether it was built for gods, kings, or long-forgotten rituals. Inside, a passage 18m (59 ft.) long leads to a central burial chamber that sits in pitch-darkness all year, except for 5 days in December. During the winter solstice (December 19–23), a shaft of sunlight travels down the arrow-straight passageway for 17 minutes, where it hits the back wall of the burial chamber. You can register for a lottery to be in the tomb for this extraordinary event, although competition is fierce—and these days it's livestreamed on the Internet, too. As part of the daily tour, you can walk down the passage, past elaborately carved stones and into the chamber, which has three sections, each with a basin stone that once held cremated human remains. As with Knowth, you should book as soon as you know the day you'll be coming—space is limited and it's one of the most popular historic sites in the country.