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"The tunnelling for Crossrail, London's new east-west rail network, is now in its most intense phase - and the scale of the engineering challenge is as jaw-dropping as the cost is eye-watering.
"The tourists and shoppers in Red Lion Street in central London can have little idea that 30m under their feet something is going on, although people in buildings with deep foundations may feel a curious tingle.
"Two tunnel boring machines (TBMs), named Phyllis and Ada, are worming their way through the earth. Each one is the length of 14 London buses and weighs 1,000 tonnes.
"A computer display in Phyllis's control cabin shows the machine is 18mm below where she should be and 27mm too far to the left. But that's all within the design's tolerance of 50mm."