If you've ever gone through a tunnel underneath a body of water, you probably wondered how on earth they managed to build it. Rivers carry a lot of water, which has a tendency to seep down into the earth. The first such tunnel was the Thames Tunnel, built under London's river in the 19th century. This kind of construction was attempted in 1799, but that was the first of a string of failures. Engineers knew how to build a tunnel, but you have to dig the hole before you build the walls inside, leaving a dangerous gap for flooding or collapse. French engineer Marc Brunel came up with a workaround: a temporary tunneling shield that could move with the diggers and builders.
Brunel’s tunneling shield consisted of a large, rectangular, grid of iron frame with 36 chambers distributed into three levels. Each chamber was open to the rear, but closed in the front with moveable boards. The front was pressed firmly against the tunnel face, and the workers would remove the boards one at a time and excavate the earth behind it to a predetermined depth. Then the board would be pushed into the hole and screwed back into place before the next one was removed. The whole process was repeated until the earth behind all the boards were excavated. Then the entire iron frame was laboriously moved forward, and the newly excavated section was shored up with bricks and mortar.
The tunneling shield was revolutionary, but the work was slow, progressing at only 8 to 12 feet a week. And although the shield worked well in preventing cave-ins, seep-ins were another problem. The filthy, sewage-laden water from the Thames above dripped down from the roof of the tunnel and poisoned the poorly ventilated space. Many miners including Brunel himself fell ill to a wide range of affliction such diarrhea, headaches and temporary blindness. Pumps worked all round the clock removing water from the tunnel, and when they failed, the whole shaft would flood to a depth of several feet.
The work was not safe, and setbacks meant the tunnel took nearly twenty years to complete, at an expense way beyond what was estimated. Read about the construction of the world's first completed tunnel under a navigable river at Amusing Planet.