"It's a real shared experience, to be in the dark and wait for the light," says Clare Tuffy, manager of the Brú na Bóinne visitor centre in Ireland's Boyne Valley.
In 2016, nearly 33,000 people entered a lottery for the chance to stand inside the Newgrange tomb on the days of the winter solstice.
Only 60 were chosen.
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On the days between 18 and 23 December, thanks to a remarkable feat of Stone Age engineering, a beam of dawn sunlight slips through an opening in the "roof box" above the entrance, then creeps along the 19-metre passage to where a hushed group of visitors waits in the blackness of the inner chamber.
For roughly 17 minutes, weather permitting, the chamber floods with light.
"There are very few experiences you can share with your ancestors across five thousand years," Tuffy says. "Five thousand years ago, people stood in this same place waiting for this same event."
Ireland's Ancient East
The Brú na Bóinne World Heritage Site, described by UNESCO as "the largest and most important concentration of prehistoric megalithic art in Europe," lies less than an hour's drive north of Dublin, at the very heart of what the country's tourism board calls Ireland's Ancient East.
Here, among the rich green hills of County Meath, stand three great mounds (Newgrange, Knowth and Dowth) along with around 40 satellite passage graves.
The valley is just as fertile now as it was when these monuments were built with tools of stone, bone and wood by a farming community in 3200 BC, some 500 years before the great pyramids rose at Giza.
The most famous of them all is Newgrange.
This grass-covered mound ringed with quartz, roughly 86 metres across and 13.5 metres high, covers an area of a single acre. Its smooth exterior hides mysterious hidden chambers within, only part of which are open to visitors.
An Eternal House of the Dead
These communal passage tombs spread right across Western Europe during the Neolithic era, linking ancient communities to the idea of immortality.
"Their own homes were very simple, ephemeral things made of wood and clay, so for the dead they built houses that would last forever," Tuffy says.
Examples still standing today include Gavrinis in France, Maeshowe in Scotland and Bryn Celli Ddu in Wales.
"As they became more and more sophisticated, they began to incorporate solar alignments," Tuffy says. They also grew in scale. At one point they were most likely the largest monuments in the world. And not without reason. UNESCO states that in the Boyne Valley the passage tomb "reached its highest expression."
"About 12% of passage tombs have a significant solar alignment," Tuffy explains. "But Newgrange is unique in that it is the only one with a special opening that lets the sun reach inside."
Temple, Tomb and Town Centre
"We think that for the people who built it, it was far more than just a tomb," Tuffy explains. "The monuments served social, economic, religious and funerary functions."
"It was a place where people gathered, a place where the ancestors were honoured. It's a symbol of a people's wealth, and probably a place where they mediated between the living and the dead."
Many of the 97 boulders (the "kerbstones") that surround Newgrange are decorated with enigmatic carvings of spirals, circles, zigzags and triangles, which may carry both astronomical and religious meaning.
In his 2012 book "Newgrange: Monument to Immortality," Irish journalist Anthony Murphy argues that Newgrange and Knowth were enormous calendars used by Neolithic farmers to measure years, leap years and much more.
After the Stone Age
"Eventually the construction became such a drain on the community's resources that people stopped building them," Tuffy says. "New ideas arrived in Ireland along with bronze. We began to see the gleam of gold, and everyone wanted to be buried in individual graves, laid to rest with fine things. So the whole tradition changed." The monuments were then abandoned, though they were still revered.
Newgrange was sealed up once its original purpose had come to an end.
And so it remained until the passage and chamber were rediscovered in 1699 by one of William of Orange's men, nine years after the troops of the Dutch Protestant William of Orange defeated the forces of the English king James II at the decisive Battle of the Boyne.
The white quartz outer facing was added in the 1970s.
While the artefacts uncovered in the Newgrange group are exclusively prehistoric, the Knowth group of 30 monuments shows "far more layers of activity," Tuffy says, with objects dating from the Neolithic all the way to the medieval period.
The archaeological complex of the Hill of Tara, ancient seat of the High Kings of Ireland and in use from the Neolithic era until the 12th century, sits a short distance from the River Boyne and can be combined with a day trip to Brú na Bóinne.
