Fire-Setting An ancient technique in which the tunnel wall is heated with fire, then cooled with water. In the right conditions, the temperature change causes the wall to break off in chunks. First used in a tunnel around 2000 B.C., to mine copper and gold from Egyptian mountains

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Roman slaves built the enormous Cloaca Maxima, one of Rome's oldest sewer tunnels, using the fire-setting technique. The tunnel was so huge and impressive, the Emperor Augustus was rumored to have toured it in a ship. Thousands of slaves died in the tunnel from the toxic fumes and intense heat, but fire-setting remained the standard tunnel digging method for well over 2,000 years!

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