Underground London by Stephen Smith

Collecting books on London is a lethal habit; you start with a couple of photographic albums and end up building shelves for esoteric handbooks on vanished railway stations. Our capital city has been burrowed into and built upon so often that discoveries are still made whenever buildings are demolished. Do we really need another book on the subject? The touchstone volume for collectors is the lavishly illustrated and somewhat mechanically obsessed London Under London (1984), but Stephen Smith broadens the brief, taking in rivers, conduits, sepulchres, subways, arches, basements, drains, societies and the quirky people who run them.

His underworld is also an underside, his book a chance "to become acquainted with the things London would prefer to keep hidden: the flaws beneath the surface, the secrets under the floorboards". Being a reporter, he's prepared to venture where few civilians would go. His plan is "to find the oldest parts of this hidden city and read the book of London backwards to the present day".

Setting off to traipse the 630 square miles of the London Underground, he realises that its weird appeal is probably rooted in it being a private rather than public space. In July 2002, a new record was set for visiting all 272 stations (it still took more than 19 hours). Smith pins down the distinctive subterranean odour ("the singed smell of an electrical retailers") before heading deeper into the supposedly impassable underground rivers. All through the book he meets people who prove that the great British eccentric is not dead. Jane, his guide to the Fleet, reveals a c-word fixation bordering on Tourette's Syndrome as she connects fertile waters with menstrual blood. Sewer-flushers raise the squeamishness-stakes with a dire warning about the bloated dead rat bobbing past them, explaining that it'll go off like a Chicken Kiev if anyone treads on it. Hard hats, boiler suits and waders are donned as Smith sploshes off into the Dantesque drain system, and as the going gets truly emetic, he captures the true stench of the city's waste in vivid prose.

But wait, we're just beginning! Poking about in the effluvia under Parliament and Hampton Court, he meets men and women whose unwholesome fascination with their environment is matched only by their knowledge of plumbing. The public can't wait to join the Thames Water tours, though one granny admits it's not everyone's idea of a fun day out. Mercifully it's not all toilets, either, and wonderful finds are revealed in unlikely places: a section of London's basilica, built in AD70, turns up in the basement of a hairdressers' shop, indicated by a sign saying "Ten hairdressing positions and shoeshine. Plus ancient monument downstairs."

Smith's investigation of Roman remains leads him to less-examined parts of Anglo-Saxon London, and the buildings that still survive. In the black museum of the Customs House, he hears tales of international bird-rustlers and heroin-packed gherkins. Over mugs of tea at Merton Abbey, he is told about a Second World War bomb that exposed the river Effra flowing beneath its crater. He even attends some of London's more peculiar ceremonies, like the beating of All Hallows' boundaries on Ascension Day, which involves dangling a schoolboy upside down above the Thames so he can thrash it with a stick.

Periodically, the lure of the lavatory proves too strong, and Smith heads back down into the poo, spotting rare netherworld mosquitos or describing Tudor thunderboxes. Historical data is slotted in effortlessly; the facts and figures appear when needed, and Smith is happy to enlist help. Scientists work out the precise amount of overkill Guy Fawkes would have created with his stash of gunpowder (he had enough to flatten Parliament 25 times over). A phonecall to the Corporation of London wins admittance to a crypt-clearance at St Andrews Church, where the team employed to lift out the corpses is running a book on how many will be found. Their wildest estimates are topped as 3,000 human remains are clocked with no sign of the crypt being emptied.

Sensing that his readers might care for a palate cleanser, Smith gives us a chapter on the higher purposes of cellars, including "rubicond sluicing" in the wine vaults of Berry Brothers & Rudd, a company who stock sack dating back to 1642. Then it's on to the silver vaults under Chancery Lane and the strongrooms where Smith witnesses a bag actually labelled "Gems" being stashed.

All through his journeys, it's clear that Smith has a positive effect on our subterranean guardians, for they unlock doors with gentle remonstrances that really, the area is meant to be off-limits... but he meets his match in a pair of bunker-investigating anoraks who creep around radar posts cataloguing masts with the faintly desperate air of people who know they're boring.

He brushes against the world of official secrets when he finds a passageway beneath the statue of George IV in Trafalgar Square that radiates out to Downing Street, the Treasury and the Cabinet Office. Sadly, Smith's exposé of hideaways such as the Citadels - four wartime fortresses - will help to seal more of underground London away. This is a shame, because so much of the centre is privately owned that access is increasingly hard to obtain. London remains a city of secrets, but as long as authors can cajole enthusiasts into dusting down the past by simply showing an interest, there's hope that we'll be able to understand our surroundings a little more. This is a benchmark in London books: elegant, illuminating and often very funny, a great guide to the dark side.