Despite two business trips to Ireland in the past three years, I hadn’t ever left Dublin when I headed there again this past June for a third time. I swore I wouldn’t make that mistake again, so booked transportation in advance to get out of the city and see a bit of Ireland’s deeper past. My goal was the Brú na Bóinne complex of megalithic monuments in County Meath, about 45 minutes north of Dublin. The centerpiece of this complex is Newgrange, a passage tomb dating from 3,200 BCE — 500 years older than the Great Pyramid of Giza.

Like pilgrims more than 5,000 years ago, my first view of Newgrange came between the trees, atop its hill across the River Boyne. Of course, I was standing in the quite modern Brú na Bóinne Visitors Centre, but the effect was still awe-inspiring.

I waited for my assigned time slot (tours of Newgrange are only available through the Visitors Centre) and walked across the river to the shuttle bus stop, pausing on the bridge to look downriver, the Boyne meandering toward the Irish Sea. It began to rain.

The edifice dominates the hill Newgrange stands on, overlooking the Boyne valley with dozens of smaller unexcavated tombs dotting farmers’ fields below. (The reconstructed exterior is somewhat controversial — did Neolithic builders have the technology to create that white vertical wall? — though what’s visible today uses all original materials.) The front of the mound is faced by a circle of standing stones that cast shadows on the entrance at key times of the year.

One of the most impressive — and photographed — external features of Newgrange is the entrance stone, carved with abstract designs such as swirls and lozenges. In the Neolithic, the stone forced ancient visitors to climb over to cross the threshold into the sacred space within. Modern visitors are afforded wooden stairs (replete with metal handrails for “health and safety”).

Photography isn’t allowed inside. This sketch from 1903 gives a sense of the passage’s general dimensions, with the main chamber at the end.

As I stepped inside, the passage floor twisted upward toward the chamber. After squeezing past stones crushed out of alignment in their walls by the pressure from 5,000 years of the mound’s weight above, I stood in the chamber. Looking up, lines of corbelled stones stepped steeply upward toward the the vaulted ceiling in the darkness.

Each Winter Solstice, the rising sun shines through an opening above the entrance and illuminates the chamber. A rainy mid-afternoon in mid-June doesn’t have quite the same light, but thanks to a little modern technology (and just a hint of blarney from our guide), I stood in the interior of a 5,000-year-old passage tomb and saw light creep across the floor and touch the rear of the chamber as it did so long ago.

It was easy to imagine how celebrants must have felt in 3,200 BCE — that connection between something we humans have made and the nature with which we’re all still a part. But there’s also a deep sense of disconnection with that past, emphasized by one little piece of information I learned as the guide talked there in the dark with a halogen light shining up the tunnel.

The sun doesn’t shine exactly on the back of the chamber. It would be easy to dismiss this little fact as a lack of precision on the part of the Neolithic engineers or astronomers who designed Newgrange. In reality, the earth itself has shifted enough on its axis over the past 5,200 years that the passage and chamber are no longer aligned with the sun. The structure is so ancient that changes in the order of the universe itself have misaligned Newgrange from the Winter Solstice sun.

We have no idea what the carvings in and around Newgrange mean. We have no idea if it was even built as a tomb, or (quite probably) some type of solar observatory connected to religious faith. Despite all we’ve learned of their material culture and environment, the builders of Newgrange remain effectively a mystery. Nothing emphasizes this more than the failure of light from our sun to illuminate the modern darkness inside Newgrange the way it did in the Neolithic.

Axial precession will bring Newgrange back into alignment with the Winter Solstice in another 21,000 years. Will Newgrange still be standing? Will we still be around to find out?