As someone who has explored sewers as a kid--they were in a new subdivision; it was on a dare--I totally understand the appeal of life underground. Who hasn't dug in their yard and hoped to find arrowheads or pottery from thousands of years ago?
Ackroyd,  who wrote a book about the above-ground city several years ago, now dives underneath to recount the other world under busy streets, cathedrals, government buildings, and flats.
It's fascinating stuff. In the 19th century workmen excavating before constructing new buildings discovered huge chunks of the Roman wall that surrounded the city about two millenia ago. Other builders during that same time period found a stairway down to a brick-walled room with a spurting spring that they believed was used as a baptismal font during medieval times.
London's massive bombing during World War II increased the amount and variety of archaeological finds. A huge ship's skeleton from Roman Empire days was discovered near the Thames. Other notable finds included Roman coins, incunabula, prehistoric flint tools, suits of armor, and lots and lots of human and animal bones including those of mastodons.
Underground is also the world of the dead, of the terrifying bubonic plague,
and although plague germs can't survive for hundreds of years, anthrax spores can and still be deadly.
Ackroyd points out that underground London also hosts the living. Runaway dogs find their way down under the tube to find warmth and food. Pigeons ride the roofs of subway cars to other stations.
If you like this book, head upstairs to read his book about the above-ground city, London: a Biography . Rachel Howard also has a fun guide book called Secret London that describes quirky tourist attractions such as The Founding Museum, Ghost Stations of the Underground, Henry VIII's wine cellar, and The Horse Hospital.