Stonehenge - one of England's most famous monuments. Here, massive slabs of stone rest on ashlars up to 8m high for thousands of years. Thousands of visitors come here year after year to see them for themselves. But the stone giants weighing several tonnes are by no means the only structures of their kind. From stone rows and circles to dolmens, megalithic tombs and menhirs, these mysterious megaliths can be found all across Europe. They have long been considered the work of giants, the devil or aliens. Nowadays, researchers know these massive stone monuments were built by Neolithic people over 4,000 years ago. However, they left no written sources behind - how and why these structures were built remains a mystery to this day. Now, researchers are close to uncovering the secret behind them.