This extraordinary prehistoric burial site was only discovered in 1968, and much of it is yet to be excavated. It is mainly composed of two massively long underground burial chambers, the longer of which stretches for 40m (131 ft.). In the mound, scientists found the largest collection of passage tomb art uncovered thus far in Europe, as well as a number of underground chambers and 300 carved slabs. Surrounding the mound, 17 satellite graves are laid out in a mysterious, complex pattern. And still, nobody can give a definitive answer to the biggest riddle of all: What was it all for? Even now, many of Knowth’s secrets have not been uncovered—excavation work is constant here, and you may get a chance to see the archaeologists digging. All tickets for here and Newgrange ★★★ are issued at the Brú na Bóinne Visitor Centre near Donore. There is no direct access to the monument; a shuttle bus takes visitors over. And as with Newgrange, it's advisable to book tickets comfortably in advance of your visit—the site can get extremely busy in summer, and space is limited.