This book focuses on the underground transportation forms that began to emerge in city centers in the second half of the 20th century. Besides providing an illustration and evaluation of 19 selected case projects, the notions of speed and time are opened for discussion as significant concepts that interact to affect the physical formations both of cities and transport policies. The developments in urban transport technologies after the 1960s are also examined as leading factors in the development of today's multimodal and multilayered urban transportation hubs. The basic questions that are to be answered in the study are: 'What were the factors that lead to the expansion of urban forms underground in the second half of the 20th century?, and 'What were the architectural and urban space properties of the underground transportation sections that have been built after the second half of the 20th century?'