The tunnel under the river at Port Huron

When a freight train at Port Huron seems to vanish into the ground near the river, it has just entered a tunnel that opened in 1891 — the first full-size tunnel ever driven under a river anywhere in North America. It still runs under the St. Clair to Sarnia, Ontario, carrying cars between the two countries below a river most people only ever cross on the bridge overhead.

Before the bore, this was the Grand Trunk Railway’s worst headache. Every railcar bound for Canada had to be floated across on ferries — slow in summer, and in a hard winter the ice could shut the crossing down for days. So the railway decided to go under instead. Crews started from both shores at once, driving a steel shield forward through the soft blue clay while men worked in compressed air to hold the river back from the open face. When the two headings met somewhere beneath the channel, they lined up almost perfectly — a remarkable feat with 1880s instruments. Portal to portal, the bore ran a little over 6,000 feet.

That first tunnel did the job for more than a century. The compressed-air work was brutal and dangerous; the bends — caisson disease — killed and crippled workers before anyone fully understood why. It was finally retired in 1994, when a wider tunnel opened alongside it, sized for the double-stacked container cars the old bore was never built to hold. The new one is what hums under the river today.

The American Society of Civil Engineers named the 1891 tunnel a historic civil engineering landmark, which puts it in company with the Brooklyn Bridge and the Panama Canal. Next time a train slips into the riverbank at Port Huron, you’re watching it follow a path engineers first cut through the mud more than 130 years ago.