Bryn Celli Ddu — 'the mound in the dark grove' — sits in a quiet field near Llanddaniel Fab, about a mile inland from the southern shore of the Menai Strait. The Ordnance Survey calls it a chambered cairn. The archaeologists call it a developed passage tomb. The Welsh have always called it what its name says: the mound in the dark grove. It is, by some distance, the single most important Neolithic monument on Ynys Môn.
The site was excavated, badly, in 1865 and properly between 1925 and 1929 by W. J. Hemp. The structure he uncovered is more or less unique in Wales. Underneath the round grass mound is a stone-lined passage about eight metres long, running roughly east-north-east, opening into a polygonal inner chamber. Inside the chamber stands a single free-standing pillar of smoothed blue limestone — almost two metres tall — that supports nothing and that was clearly placed there for its own sake. Behind the chamber, embedded in the cairn material, Hemp found a slab carved with serpentine, spiralling lines: the so-called 'Pattern Stone,' a near-relative of the carvings at Newgrange and Knowth in the Boyne valley. The original is now in the National Museum in Cardiff; a cast stands in its place at the back of the tomb.
But the thing that has made Bryn Celli Ddu world-famous in the last fifty years is the alignment. Around the summer solstice — for about a week on either side of 21 June — the first light of the rising sun strikes through the entrance passage of the tomb at exactly the right angle to run the full length of the corridor and to light the back wall of the inner chamber. The effect lasts only a few minutes. It is reliable enough that hundreds of people now travel to Anglesey every June to stand in the cold field at four in the morning and watch it happen.
Long before the archaeologists noticed the solstice alignment, the people of Llanddaniel Fab had their own opinion about Bryn Celli Ddu. The mound, they said, was a porth — a gate. On the right nights of the year, with the right kind of mist on the Menai, it opened into Annwn — the otherworld of Arawn, the place from which the Cŵn Annwn hunt out and to which Pwyll Pendefig Dyfed travelled in the First Branch of the Mabinogi. People who slept on the mound at midsummer were said either to wake with the gift of cyfarwydd-craft — the storyteller's art — or not to wake at all.
There is a slight, real archaeological echo of the folklore. The Neolithic builders of the passage tomb were preceded on the same site by an earlier henge — a circular bank-and-ditch enclosure with a ring of standing stones inside it. They demolished it, levelled it, and built their tomb on top of the ground their predecessors had marked as already sacred. Bryn Celli Ddu has been a doorway, in one form or another, for about five and a half thousand years.
Wear something inspired by Bryn Celli Ddu and you wear the oldest engineered piece of darkness on Ynys Môn — a tomb shaped to admit one ray of midsummer sun, and an old Welsh rumour that on the right night it admits rather more than that.