Exploring London's 'smart' tunnels
Crossrail - a new rail network buried under London's streets - is the largest infrastructure project in Europe and when completed it is expected to carry around 200 million passengers a year.
But the city already has a huge network of underground tunnels - some are almost 100 years old - and engineers need to ensure that work on Crossrail does not disturb them. Damage to an older tunnel could affect structures on the surface.
The BBC's Dave Lee finds out how groundbreaking sensors are helping to monitor any movements and allowing engineers to understand how London's network of underground tunnels are being affected by the new rail project.
- From the section Technology