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There are many components to underwater subway construction, but the key is the tunneling shield, a machine that made digging through water-bearing strata both safe and possible. The shield was pioneered in 1818 by Marc Isambard Brunel, a French naval officer living in England who noted how shipworms bore through wood, pushing sawdust behind them with the aid of their shell armor. Putting his observation to practical use, he built a giant iron shield that could push through soft ground while miners shoveled the dirt through shutter openings in the face. Brunel's technique was used in
constructing the world's first underwater tunnel, built underneath London's River Thames and completed in 1843. Consequently, one of engineering's long-standing obstacles was removed, and similar devices were employed throughout Europe and the Americas. Perhaps the best known underwater subway today is the Channel Tunnel or "Chunnel," which connects Great Britain and France. Construction crews from the two nations worked together for three decades, employing powerful steel cylinders, hydraulic jacks, and air compressors to burrow under the waterway. The tunnel lining was completed by setting steel rings in place. The steel was later covered with masonry. So next time you travel on an underwater subway, thank the industrious little shipworm for inspiring the ride.
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