Early in the morning in Texas a vehicle slows and turns onto a side road. Some 7,6ookm (4,700 miles) away in Farnbo-rough, south-west of London, a series of red lines zigzag across a computer screen and raise an alarm. That the lines resemble those on a seismograph picking up a small earth tremor is entirely appropriate: these lines represent tiny ground vibrations made by sounds from above. More lines appear as the vehicle rumbles across a cattle grid, identifying it as a two-axle vehicle, probably a car. Then it stops. Subsequent lines are interpreted as footsteps followed by the sound of digging. A security camera swings round to take a look-and a man with a spade cheerfully waves back. This time it is just a demonstration.
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