Stone alignments at Carnac in Brittany, where more than 3,000 stones comprise the largest of Europe's megalithic monuments. Many have been lost, and the original total was probably more than 5,000 stones. They likely date to around 3000 BC. Nobody knows what they were for, precisely, or how so many came to be erected. One theory is that setting up a stone came to be an annual ritual for many families or communities in the region.