Underground London: Travels beneath the City Streets. Stephen Smith. Little, Brown. [pounds sterling]17.99. ix + 390 pages. ISBN 0-316-86134-0. The author of this travel book cum popular history of London looks at the capital and its people from the inside out. His concern is with the unseen world and he begins with an exploration of the sewers. He moves on to tunnels (used and unused), discarded Underground stations, rivers such as the Fleet and Tyburn which have been covered over for years, archaeological sites and underground structures, such as the secret location of the Cabinet War Rooms. One of the most interesting chapters is based on a tour of the Mount Pleasant sorting office and contains a discussion of the underground railway system created by the G.P.O. in the 1860s to carry post round London. Another deals with the Thames Barrier and the unending problem which London’s engineers face when dealing with water. He reminds readers that the Thames was at one stage three times its present width. He points out that ‘in the end, it came down to water, to water and clay. That’s what London was made of’. He uses his journalist’s training to good effect and gives a vibrant account not just of physical structures but of the men who maintain them. (T.P.M.)