Bryn Celli Ddu burial chamber.

One of Anglesey’s most famous prehistoric landmarks, Bryn Celli Ddu (the ‘Mound in the Dark Grove’ in English) is actually two sites in one. In the early Neolithic period, a henge (bank and ditch) enclosing a circle of stones was built here, to be replaced later by a chambered tomb beneath a mound measuring up to 26m in diameter. Inside, a long, narrow passage leads to an octagonal chamber 2.4m across, where artefacts such as human bones, arrowheads and carved stones have been found.

But Bryn Celli Ddu’s most unusual feature can only be seen once a year. As the sun rises on the summer solstice (the longest day of the year) shafts of light shine directly down the tomb’s passageway to illuminate the chamber within.