Channel Tunnel

For two hundred years the English Channel was the reason Britain felt like an island. Since 1994 it hasn't quite been one: a 50.46 km railway runs from Folkestone in Kent to Coquelles near Calais, and for most of that distance the trains are rolling through chalk marl with the sea pressing down overhead.

The crossing

The tunnel links Folkestone (UK) and Coquelles (France) over a total length of 50.46 km. Of that, roughly 37.9 km runs beneath the seabed — the longest continuous undersea stretch of any tunnel anywhere. At its lowest the rails sit about 75 m below the sea floor, deep enough to stay inside a single band of soft, waterproof chalk marl that the engineers deliberately followed the whole way across, like threading a needle underground.

Three tunnels, not one

What people call "the tunnel" is actually three parallel bores driven side by side:

• Two running tunnels, each 7.6 m in diameter, spaced 30 m apart — one for each direction of travel.
• A smaller central service tunnel, 4.8 m across, sitting between them.

The service tunnel is the safety heart of the system: it stays at higher air pressure to keep smoke out, gives emergency crews a protected route to any point in the crossing, and connects to the running tunnels through cross-passages. Piston-relief ducts also link the two rail bores so a train doesn't push a wall of pressurised air ahead of it.

Boring under the sea

The tunnel was dug by eleven tunnel-boring machines — six starting from the English side, five from the French — grinding toward each other from both coasts at once. Their triumph was meeting in the middle, under the sea, with the two halves lining up almost perfectly. It opened on 6 May 1994 after years of work and heroic cost overruns, one of the largest privately financed construction projects of its era.

What runs through it

Two very different kinds of train share the rails:

• Eurostar passenger services, part of the high-speed rail network — trains built to cruise at 300 km/h on the open line at each end, though they slow to a regulated pace inside the tunnel itself for safety.
• Le Shuttle, the drive-on car-and-lorry ferry-on-rails: you park your car inside a wagon, the doors close, and 35 minutes later you drive off in another country. Freight trains use the tunnel too.

Longest undersea — but not longest overall

Here the superlatives need care. The Channel Tunnel holds the record for the longest undersea section, but it is not the longest rail tunnel in the world. That title belongs to Switzerland's Gotthard Base Tunnel at about 57 km, which runs entirely under mountains, not water. Japan's Seikan Tunnel is also longer end-to-end. So the Channel Tunnel's claim is specific and unbeaten: no tunnel on Earth spends more of its length beneath the sea.