In a quarry at Baalbek, in modern Lebanon, lies “the stone of the south” — a single hewn stone weighing 1500 tons. It’s not surprising that the ancients abandoned it; if they’d got it out of the quarry it would have been the largest stone ever moved.

But a retaining wall nearby contains a row of three stones weighing 750 tons apiece. Somehow they were raised 22 feet into position. No one knows how.