In ancient Rome, an hour was not a consistent unit of time. In the summer, it could be as long as 75 minutes – and in the winter it sometimes lasted just 45 minutes. At first it just appeared to be a plain block of carved stone. The limestone lump was lying face-down in the mud at the site of an ancient Roman town in central Italy, and it was thoroughly stuck.

It’s thought that the block was stolen in the medieval era – plucked from the metropolis' antique remains and dragged away, possibly with the intention of using it as building material. But the mud had thwarted this attempt, and here it was, still in position, hundreds of years later.