31 miles of engineering beneath the English Channel. One of the greatest construction projects in human history.
The Channel Tunnel actually consists of three parallel tunnels:
Cross-passages connect the service tunnel to the running tunnels every 375 metres. In an emergency, passengers can be evacuated through these passages into the service tunnel, which maintains its own air supply and is accessible by road vehicles.
The tunnel was bored through a layer of chalk marl — a soft, impermeable rock that sits beneath the English Channel seabed. This geological layer was critical to the project's feasibility. It's soft enough to bore through efficiently, dense enough to be structurally stable, and watertight enough to prevent flooding.
Finding and staying within this chalk marl layer was one of the project's great engineering challenges. The tunnel follows the layer's contours, dipping and rising to remain within the optimal geological zone.
The idea of a Channel crossing dates back centuries. Napoleon considered a tunnel in 1802. Serious engineering proposals emerged in the 1870s, and a small pilot tunnel was actually started in 1880 before being abandoned due to British military concerns about invasion.
The modern tunnel was finally agreed upon by the British and French governments in 1986. Construction began in 1988 with tunnel boring machines working from both sides. The historic breakthrough — when the two halves met in the middle — came on 1 December 1990. Four more years of fitting-out followed before the tunnel opened for commercial service in 1994.
The Channel Tunnel carries over 20 million passengers per year through a combination of Eurostar passenger trains and Eurotunnel Le Shuttle car/lorry services. It's one of the most heavily used rail tunnels in the world and an enduring symbol of European engineering cooperation.