The world’s most famous prehistoric monument continues to surprise and amaze. Researchers have revealed that the long-mysterious “Altar Stone” at the heart of Stonehenge came from faraway Scotland, raising tantalizing new questions about how — and why — a six-ton slab of sandstone made its way from north to south 5,000 years ago.

Could it have been deposited nearby by ice-age glaciers that covered Britain during the woolly mammoth Pleistocene? Probably not. The ice sheets in Scotland were moving north, not south, and so deposited their moraines in the opposite direction.

So we are left with two theories: that the Altar Stone was dragged 500 miles or more overland by our Neolithic forebears, before the invention of the wheel, across the high hills and through the dense forests of prehistoric Britain; or, as these scientists speculate, that the stone was ferried via sea by Stone Age mariners, a routing that “demonstrates a high level of societal organization with intra-Britain transport during the Neolithic period.”

This means that Neolithic peoples were capable of delivering a Stone Age prehistoric FedEx package weighing 13,227 pounds from as far north as the Orkney Islands off the coast of Scotland to the Salisbury Plain southwest of present-day London, with the understanding that six tons is approximately the weight of a full-grown male African elephant. Such a trip might have taken a decade or more — but “just in time” is relative.