Beneath the visible surface of cities lies an invisible architecture. Subways, tunnels, water systems, data cables, and bunkers form a dense network that sustains urban life while remaining largely unseen. The ground beneath our feet is not a void but a complex territory that holds the infrastructures, memories, and anxieties of our age. In recent years, as land becomes scarce and climate pressures intensify, architects and urbanists have turned their gaze downward, rediscovering the subterranean as both a physical and conceptual frontier. To design underground is to engage with the unseen mechanisms that shape the world above.