Choose another wonder
Location: London, England
Completion Date: 1843
Length: 1,200 feet
Engineer(s): Sir Marc Isambard Brunel
By the early 19th century, London, England was a thriving city. Several bridges crossed the Thames River and more were needed, but construction of a new bridge would have brought ship and ferry traffic to a standstill. The British were rooting for a new structure: a tunnel under the Thames River. Unfortunately, the tools of the day -- explosives and power drills -- were no help for building tunnels through soft, watery ground at the bottom of most rivers. Several attempts had been made to dig a tunnel beneath the Thames River, but they were all spectacular failures. It wasn't until 1825 that a French engineer named Marc Isambard Brunel finally found a way to do it.
Brunel invented the tunnel shield, a giant iron box that could be pushed forward through soft, gooey soil. Diggers worked from 36 individual cells in the box and faced a wall of removable wooden planks. Each digger removed one plank at a time, scooped out about four inches of muck, then quickly replaced the board. The shield was pushed forward by hydraulic jacks, and the whole tedious process was repeated. While the iron shield held up the gooey soil, workers lined the tunnel walls with brick.
But as the tunnel progressed, so did its problems. The wooden planks were too weak to support the soft, watery soil, and the entire tunnel flooded five times. Methane and other pollutants in the soil caused unexpected explosions -- and deaths -- in the tunnel. Finally, 18 years after construction began, Brunel's tunnel shield emerged on the other side of the Thames, proving for the first time that it is possible to carve a tunnel underwater.Here's how this tunnel stacks up against some of the longest tunnels in the world.
(total length, in feet)