How did Brunel's iron shield let miners dig under the Thames when engineers said it was impossible?

Marc Brunel made it possible by inventing the tunnelling shield: a big iron frame that held up the soft ground in front of the diggers and protected them from cave-ins. Before him, people had tried to dig under the River Thames and failed. A group of Cornish miners were used to hard rock and did not change their methods for the soft clay and quicksand of the Thames, and when their attempt failed, engineers concluded that an underground tunnel was impracticable.

Brunel refused to accept that conclusion. He patented his shield in January 1818, and the story goes that he got the idea from the shipworm, a mollusc that bores through submerged timber. The first shield was a large rectangular scaffold-like iron structure with three levels and twelve sections per level. Its key innovation was its support for the unlined ground in front and around it, to reduce the risk of collapses. Brunel and his son Isambard used it to build the tunnel between Rotherhithe and Wapping, in London.

It was a dreadful job. Workers fell ill from sewage-laden water seeping in from the river, and the sewage gave off methane that the miners' oil lamps ignited. Work was slow, only 1 to 8 feet a week, so the company let sightseers come to watch the shield in operation. On 18 May 1827 the tunnel flooded, and Isambard lowered a diving bell from a boat and threw bags of clay into the breach in the roof. Afterwards he held a banquet inside the tunnel. In 1828 it flooded again, and six men died.

Finished in 1843, it was the first tunnel known to have been built successfully under a navigable river, and it showed that underwater tunnels could be built after all. It is still in use: since 1869 it has carried trains.