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.