How the Channel Tunnel Was Dug Under the SEA Without Flooding

They dug a tunnel under the open sea, and the miracle wasn't the machine. It was one thin layer of rock.

With the whole English Channel pressing down overhead, how did the tunnel not simply flood?

Deep under the seabed sits a band of rock called chalk marl, a soft, clay-rich chalk about twenty five to forty five meters thick. It is soft enough to dig through quickly, but packed with clay that makes it nearly waterproof, so seawater does not seep in. Geologists mapped that layer and the machines stayed inside it the entire crossing. Too high, and they would hit cracked, water-bearing chalk that could flood. Too low, and they would hit unstable clay that could collapse. So the tunnel snakes up and down to hug that one safe band.

Here's the strange part: the tunnel is not straight or level. It rises and dips across the whole seabed, following the geology, because the engineers trusted the rock more than a straight line.

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