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 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.