"We don't have an owner's manual, and there is no writing," he noted.
However, archaeologists have pieced together some of the story.
First and foremost, the passage tomb honored the dead, most likely the high-powered dead, Krupp said.
But astronomy was also important.
"It is very deliberately designed and constructed to capture the light of the rising sun at the winter solstice, to allow that beam of light to fall on the innermost chambers of it—a place where in fact the remains of the honored dead were incorporated," he said.
Scratch marks in the window above the door indicate that rocks were repeatedly removed and put in place to open and close the window, suggesting a regular gathering at the monument for a winter solstice ritual.
"The winter solstice is a crucial moment, in that it marks the time the sun has reached the depths of winter—its darkest moment, its death, [and] its rebirth," Krupp said.
Today as many as 200,000 people a year come to view the ancient mound, making it the most visited archaeological site in Ireland. Access to the monument is controlled by the Brú na Bóinne Visitor Center.
The solstice is the most sought-after time to visit the monument. So in 2000 the visitor center switched to a lottery system for tickets, deeming luck-of-the-draw fairer than a ten-year-long wait list.
Schoolchildren pick the winners in late September or early October. For five days around the winter solstice, 20 people a day are granted access to the chamber at sunrise.
And on the day of the actual winter solstice—usually December 21—several hundred people also gather outside Newgrange to watch the sunrise.
But that's far fewer than the thousands that gather for the summer solstice sunrise at Stonehenge.
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