It may be 26 years since the first borers met in the middle, but still few engineering projects have been as ambitious as the Channel Tunnel. Surveyors, used to plotting courses above ground, were tasked with mapping the unseeable, three-dimensional space beneath the seabed. They dropped probes down 12 boreholes in the congested Channel and measured the speed of soundwaves passing through layers of rock and soil to find the impervious blue chalk stratum, which geophysicists determined would protect the tunnel from the water table above.
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