Under the surface of cities and within build environments lay hidden layers that for some play a key role in the urban scenario when others got lost in transformation. During the 20th Century, both theoretical works and technical innovations gave rise to several massive urban underground projects in New York, London and Paris, directly contributing to the definition of the new profession of urban planner. This underground planning and development in major cities was influential to a discussion on whether the city's future was going to be above or underground. Underground planning therefore corresponds to a key moment in modern urbanism and architecture, both in the East and the West. The dimensions of the infrastructure and large complex facilities that resulted from this time do not allow us to ignore them simply because they do not correspond to our sensibilities any more. How do we learn how to develop projects in these underground built environments? This book proposes to redeploy these substantial structures and their potential. Looking at urban and architectural strategies from their basement stratifications up reveals a rich cultural background that is crucial to understanding of the metropolitan development in cities like London, Paris, New York, Stockholm, Toronto, Istanbul, Moscow, Abu Dhabi, Montreal and Houston. Such a topic brings design and theory together in a renewed paradigm, as massive underground transformations are about to take place, for instance in the Paris metropolitan region at the occasion of the Grand Paris new subway network. The questions set by the underground confront simultaneously several fields such as urbanism, architecture, public space, infrastructure, landscape, night-time cultures etc. Illustrated by the innovative work of the award-winning French Practice AWP, this volume asks questions such as: how should we rethink underground parking, in respect to less car use? and where should the dead be buried in the future? With the