The connections leading to underground transit lines have not received the attention given to public spaces above ground. Considered to be merely infrastructure, the design and planning of these underground passageways has been dominated by engineering and capital investment principles, with little attention to place-making. This underground transportation area, often dismissed as “non-space,” is a by-product of high-density transit-oriented development, and becomes increasingly valuable and complex as cities become larger and denser. This dissertation explores the design of five of these hidden cities where there has been a serious effort to make them into desirable public spaces. Over thirty-two million urbanites navigate these underground labyrinths in New York City, Hong Kong, London, Moscow, and Paris every day. These in-between spaces have evolved from simple stairwells to networked corridors, to transit concourses, to transit malls, and to the financial engines for affordable public transit. The Oculus in New York City makes connections to the World Financial Center, the new buildings on the rebuilt World Trade Center site, and a new transit hub. The Jubilee Station in London brings people to the heart of the London Docklands, fast becoming a major new urban center. The Central Station in Hong Kong unites multiple levels of intense development which has also helped finance the transit. The Okhotny Ryad in Moscow, built in palatial style as a deliberate statement of the rights of the proletariat, has now been modernized by the addition of retail. The Les Halles station in Paris has made underground space a destination. This dissertation traces the historical development of each of these underground systems and documents them in detailed drawings so that their spatial structures can be compared. The way people use these places has been documented with photographs and maps using participant observation methodology. By analyzing these cases, we can identify a generalizable theory for reinventing underused subterranean public spaces as effective places, realizing the potential for rich public life, so often obscured by the heavy foot traffic in transit hubs. Underground public spaces need not be gloomy and unsafe, and the vitality of public space is determined not only by its location, but by the design of its physical setting. These five hidden cities are success stories showing how designers and users have converted non-spaces into places through design and active civic engagement.