In ways both literal and metaphorical, much of the important action in this book takes place underground. Most obviously, there is the London sewage system that Joseph Bazalgette designed in 1858, and that Johnson compares to a more famous, public monument such as the Eiffel tower. However, this is not the only moment in the book when Johnson stresses the importance of buried or hidden events, as opposed to more traditionally celebrated ones. Writing of epidemic victims in Chapter 2, he observes that—unlike more traditional participants in history, such as army generals and presidents—epidemic victims are generally unaware of their public significance. They are simply going about their daily lives, with no knowledge of what is about to befall them. They are therefore unwittingly creating a “history from below”(32), one that will not be discovered until long afterwards.

John Snow’s “ghost map”—which also gives the book its title—is of course a record of this discovery. It is a map of the 1854 cholera victims—who are represented on the map by black bars—and Johnson draws a link between it and our internet maps of today, suggesting that Snow’s map anticipated Google maps in its intense locality and specialization.