If you're something of a daredevil, and further, if you've ever wondered what it'd be like to climb to the top of the Brooklyn Bridge --- or wander amid the catacombs beneath Paris, or maybe just take an up-close look at a "ghost station" within the far-reaching New York City subway system --- you might be a latent "urban explorer." Our guest on ST is an active explorer of this sort; Moses Gates, who joins us by phone, is also an urban planner, a licensed New York City tour guide, and an assistant professor of demography at the Pratt Institute. He tells us about his new book, "Hidden Cities: A Memoir of Urban Exploration," which also describes the worldwide network of loosely affiliated subcultures dedicated to urban exploration: certain people, from various backgrounds and walks of life, who create secret art galleries in train tunnels, break into national monuments for the "fun" of it, and travel the globe sleeping in centuries-old underground labyrinths or abandoned relics rather than in hotels or inns. As the writer and public-radio storyteller Davy Rothbart has noted of this work: "[It] offers a thrilling glimpse into the secret worlds that surround us. Moses Gates has crafted an endlessly absorbing book that succeeds on many levels --- as a compelling travelogue, a nuts-and-bolts how-to manual, and a deep-feeling and highly relatable personal memoir. Anybody who reads it will emerge invigorated by possibility."