4With over 60 per cent of the world's population living in cities, the networks beneath our feet - which keep the cities above moving - are more important than ever before. Yet we never truly see how these amazing feats of engineering work.

Just how deep do the tunnels go? Where do the sewers, bunkers and postal trains run? And, how many tunnels are there under our streets? Each featured city presents a 'skyline of the underground' through specially commissioned cut-away illustrations and unique cartography.

Drawing on geography, cartography and historical oddities, Mark Ovenden explores what our cities look like from the bottom up.