Today, more than half of the world's population lives in urban areas with a projected growth to over two-thirds in 2050. The city-state of Singapore faces the challenges of supporting a sustainable urban system with a growing population in a limited land, sea, and air space. The only direction to develop is toward the subsurface. Conventional geophysical methods are not applicable because of their disruptions to urban activities, a deteriorated performance due to strong anthropogenic interference, and unsatisfying spatial and temporal resolution. In this seminar, we present recent advances in passive sensing to harvest the urban noise from traffic, construction, and ocean waves with small, dense arrays. We design novel signal processing techniques that turn urban hum into rich information of the urban system, both above and below the surface. The resulting meter-scale spatial resolution and minute-scale temporal resolution are the cornerstones to meeting engineering demands in urban environments. We present newly-developed fiber sensing technology using existing dark fiber infrastructure that has the potential to enable a million-sensor system underneath each major city around the globe. Applications of these techniques in shallow bedrock mapping, deep aquifer identification, and near-surface monitoring for geohazards provide the opportunity for geophysicists to contribute directly to urban society in planning, managing, and monitoring its space, water, infrastructure, and other resources.