Bryn Celli Ddu - a prehistoric site on the Welsh island of Anglesey located near Llanddaniel Fab. Its name means 'the mound in the dark grove'. The Summer Solstice sun lights up a quartz stone at the back of the chamber.
Arthur's Stone, Cefn Bryn, Gower, WALES. The legend is that Arthur found a pebble in his shoe in Llanelli (on the opposite coast) and threw it away so that it landed on the top of Cefn Bryn! In reality, the capstone of Arthur's Stone is a natural boulder, probably glacial, and there are two chambers underneath. (Cont. reading below in the comments. Click photo for full size.)