A worker crawls through the spokes of Tunnel Boring Machine (TBM) Victoria, used in London’s Crossrail project © Adrian Dennis/AFP/Getty Images

How do tunnelling machines know where they are?

There's no horizon to get your bearings when you're going deeper underground.

Asked by: Nigel Wight, Exeter


To keep 1,000-tonne, 150m long Tunnel Boring Machines (TBMs) like those used for London’s Crossrail project on track, engineers rely on a laser-based system. Precise reference points are set up below ground behind the TBM, and laser beams are sent out from them into receivers in the machine. This keeps the TBMs heading in the right direction to within a millimetre or so over distances of up to 100 metres.


Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.