Crossrail is tunnelling 73 miles of tunnel underneath London.
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Drone's view of Crossrail tunnel

12 March 2015 Last updated at 12:07 GMT

Watch this incredible video captured by a drone showing off Europe's biggest building project underneath London.

Crossrail is tunnelling right through the heart of London.

They'll create 73 miles of railway line.

The BBC's Richard Westcott was allowed to take a peek.