Peter Ackroyd's massive, energetic LONDON: A BIOGRAPHY was a huge hit when it was published a decade ago, outlining the strange and fascinating history of this sprawling, ancient city. It was also a huge book, weighing in at over 800 pages. Now Ackroyd follows up that blockbuster with a slim but no less fascinating volume on "the secret history beneath the streets."

"Anyone who reads LONDON UNDER will come away from the book with their own vision of this historic city both deepened and transformed."

As Ackroyd notes, London, whose civilized roots stretch back to the Roman settlers and beyond, is built on anything but a firm foundation; in fact, the city has been sinking for centuries. In some places, floors that are now at street level would have been second-story rooms when their structures were first built. What's more, in an attempt to maximize living space --- or, in many cases, to ward off evil spirits or very real diseases --- the several small and large rivers that used to course through the city's neighborhoods have, over the course of centuries, been rerouted underground.

London's remarkable Underground subway system is profiled here, along with the men who built its tunnels --- often at their own great peril. So are less glamorous underground city workings, including water pipes and sewers, as well as glimpses of the subterranean monsters and ghosts of Londoners' imaginations.

While Ackroyd takes readers on this unexpected layer (or many layers) of London that few think about and even fewer see, he also ranges back and forth freely through history, offering glimpses of London's pagan origins and early Christian shrines as well as its more recent history. He also peppers his own quite evocative prose with quotes from Dickens, H.G. Wells, and other authors.

At times, Ackroyd's rapidly shifting focus can seem disorienting, especially for those who have not internalized a map of the city's streets and neighborhoods (since the book doesn't include a map, a copy of London A to Z at one's elbow can be very helpful when trying to follow the routes Ackroyd traces of subway lines or underground streams). Certainly, those who will take most away from the book will be those who are already intimately familiar with London above ground. For the rest of us, it's also possible just to sit back and enjoy, watching his robust vision of the city flow by like a rapidly churning underground stream.

Ackroyd can get downright poetic when he's writing of these hidden worlds. About the Underground he writes: "like the escaped prisoner yearning for his dungeon, I often dream of the Underground. I dream of lines going to improbable destinations all over the world. I dream of strange encounters on platforms with people I seem to know. I dream of coming up for air and being confronted by a transformed cityscape." Perhaps the city transforms itself so quickly only in Ackroyd's dreams, but the fact of the matter remains: Anyone who reads LONDON UNDER will come away from the book with their own vision of this historic city both deepened and transformed.