Bryn Celli Ddu is a prehistoric site on the Welsh island of Anglesey located near Llanddaniel Fab. Its name means 'the mound in the dark grove'. It was archaeologically excavated between 1928 and 1929. Visitors can get inside the mound through a stone passage to the burial chamber, and it is the centrepiece of a major Neolithic Scheduled Monument in the care of Cadw. The presence of a mysterious pillar within the burial chamber, the reproduction of the 'Pattern Stone', carved with sinuous serpentine designs, and the fact that the site was once a henge with a stone circle, and may have been used to plot the date of the summer solstice have all attracted much interest. The monument Bryn Celli Ddu is generally considered to be one of the finest passage tombs in Wales. Its passage and burial chamber are complete, and it is still buried under a mound or cairn, reinstated following its excavation in 1929. (Many stone chambered cairns have lost these features.) As it now stands, the passage is long, the first being unroofed with a pair of portal stones. The main passage has walls of vertical rock slabs, roofed by a series of stone lintels. The mound, being substantially smaller than as originally made, no longer completely encloses the burial chamber, so the back wall is open to the air, allowing some natural light in. Free-standing inside the burial chamber is a smooth pillar of blueschist, a metamorphic rock, some high, with a very rounded shape. Beyond the back wall of the chamber, in a location that would once have been within the mound, is a replica of the 'Pattern Stone'. This was found buried under the mound, and has been put standing up in what is thought to have been its original location at a time when the site was a henge rather than a tomb. The patterns take the form of sinuous serpentine shapes that wind around both sides of the stone. Inside the tomb another stone has a small spiral pattern chipped into it, although its authenticity has been questioned. Outside the tomb, a ring of kerbstones shows the original extent of the mound, and they also follow the line of the ditch of the earlier henge monument. Three of the stones, visible within the cairn mound, are thought to be from the stone circle of that time. The passage is roughly