ISOCARP and ITACUS joined forces to commission five case studies on the use of underground space beneath our cities. This has led to five very different studies ranging from the identification of long lost spaces beneath the city of Naples to purpose-built caverns in Hong Kong. One case study looks at how it was possible to build a metro in Athens and not destroy the archaeological assets buried in the soil beneath the city. If you were to compare New York City and London, what can you say about the use of underground space and the key factors governing this development? One of the case studies does just that and comes up with interesting conclusions. The fifth case study looks into how underground space planning has been utilised in the Chinese city of Tianjin.