A correspondent recently introduced us to the so-called Wonders of the World, which turned out not to be dazzling great sites from antiquity but rather exemplars of design and engineering from the last two hundred years. Among them was Paw-Paw tunnel, where the Chesapeake and Ohio canal cuts through the Allegheny mountains in the USA. As it happens, we had never heard of Paw-Paw tunnel, being afflicted by virtue of our calling with a certain European myopia. But news of this 950 metres long American wonder impelled us to check out Europe's canal tunnels.