Archaeologists in Wales have used 3D digital modelling to shed new light on a strange prehistoric mound that aligns with the summer solstice. The 5,000-year-old Neolithic passage tomb known as'Bryn Celli Ddu' in Anglesey includes a stone cemetery that was important for prehistoric people for thousands of years. Using ground-penetrating radar, researchers have found rock art and monuments that date back thousands of years - suggesting this burial complex was much bigger than previously thought. Known as one of the most important archaeological sites in Britain, Bryn Celli Ddu was once constructed to protect and pay respect to the remains of ancestors. It was first discovered in 1865, reconstructed in 1920s and excavations in the last few years they have uncovered a prehistoric burial cairn above the ground.