Something like 20% (IIRC) of the entire world’s seaborne freight goes through the Channel. Amazing but true.
It is the busiest sea lane in the world. With strict lane discipline, despite which you do NOT want physical structures in the way of so many large ships!
ETA @GulliverFoyle beat me to the point.
Is pressure higher underground? I know that the ‘weight’ of water makes pressure higher at greater depths of water, but the chunnel is through rock, not water. Although the top is close enough to the ‘surface’ of the rock and the water at some points that some special measures were needed, I seem to recall. Ah … thinking about it, I guess I just indirectly made your point - you mean ‘the pressure on the rock above the tunnel would be huge’.
