Tim and Siddy are deep beneath Clapham Common; in fact, they're so deep, the Northern line is running above their heads. This is Clapham South deep level shelter, an enormous, purpose-built air raid shelter. During the Second World War, as rockets landed on the streets above, thousands of people could sleep down here in more than a mile of tunnels - all built by London Transport in the same way they dug tube tunnels.

Tim Dunn and Siddy Holloway have a night-time adventure as they go on a late-night exploration of two of London's least known stations: St Mary's and the original Aldgate East, they're only a few hundred metres apart but trains haven't stopped at either since 1938.

Tim and Siddy explore the hidden parts of Euston underground station - and there's a lot to see, including some features not found anywhere else on the London Underground.

Tim and Siddy explore the disused parts of Piccadilly Circus.

Siddy takes Tim on a mystery tour of a secret station deep beneath Hampstead Heath.

Tim Dunn and Siddy Holloway explore the abandoned branch line between Holborn and Aldwych, closed in the 1990s. And at Holloway Road, they find out about the Tube's only spiral escalator!