This week, we’ve already taken you on the first passenger journey on Crossrail, but what else is going on beneath us in the capital?

London's new £4.2 billion Thames Tideway Tunnel, or ‘super sewer’, is due to open in 2025 to help cope with capital's overflowing waste water.

With the existing system, when there’s heavy rain, sewage overflows into the River Thames, polluting the water and killing marine life, which happens up to 60 times a year.

We take a trip down into one of the 70-metre deep project’s cavernous bores, at its Battersea riverside access shaft, to meet project manager Ignacio Tognaccini - and hear some surprising subterranean music.

In today’s episode we’ll also be looking at the challenges of digging through London’s damp clay while avoiding hundreds of years of other infrastructure projects, and making sure these huge bores don’t collapse in on themselves.

Plus we’ll look at the top secret stuff below the pavements, where maps show only a grey void and government telephone number.

The Leader speaks with Martin Knights, a tunnelling engineer who chairs construction firm London Bridge Associates, about modern digging processes and the challenges of avoided existing, centuries-old buried infrastructure.

We also meet infrastructure expert Liz Reynolds, an urban planner for Hackney-based studio called Tapestry and former town planning advisor for Crossrail’s central stations.

She discusses why it’s so tricky to find the best spot to dig and looks at early tunnels, such as the Brunel-built Thames Tunnel linking Wapping to Rotherhithe in the early 1800s.

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