How did the Mont Cenis Tunnel cross the Alps?

In The World of Wonders, the Mont Cenis Tunnel is a railway passage driven through the Alpine barrier between France and Italy, intended to replace a route over the mountain that snow could interrupt and that the book describes as uncomfortable or dangerous^[1]. The account gives its length as 12,220 metres, or about seven and a half miles^[2].

The work began in November 1860 and the tunnel opened publicly on September 17, 1871^[3]. Crews bored from both sides through rock the book identifies chiefly as crystallised calcareous schist, with masses of quartz^[4][5]. Working from opposite ends saved time, but made alignment crucial: the bores had to meet inside the mountain^[6]. The book reports that a final summary of the surveying results found a deviation of less than a foot over the full distance; it does not give a separate measurement of the crews’ meeting point^[7].

The finished tunnel was lined throughout with locally quarried stone^[8]. Its entrances were not level: the French entrance stood at 3,946 feet above sea level and the Italian entrance at 4,380 feet, a difference of 434 feet^[9]. These are the nineteenth-century book’s figures and account of the achievement, not an independently vetted modern assessment.