The subsurface is a technical space, the "engine room of the city, " that incorporates the vital functions of water and energy supply, communication systems, sewers and drainage. Natural systems too -crucial for stable, dry, cool and nature inclusive cities -are also largely dependent on the quality of the subsoil. The subsurface is critical in an era of climate and demographic changes, the energy transition and economic uncertainty and constraints. However, due to the domain's current segregation and a weak urban design and planning connection, crucial design potential, benefits and innovations, remain unexploited. This paper first introduces an innovative systems approach, the System Exploration Environment and Subsurface (SEES), to recognise the subsurface as an "underarching" domain for urban planning and design. The physical impact of the subsurface on the surface quality is described for the categories: civil constructions, water, energy and soil/ecology. After setting understanding of the surface and subsurface as one united space, the paper will go into using the SEES as knowledge brokerage tool, integrating the subsurface artefacts into the design process and how the concept of Reversed Engineering with Nature is useful to uncover synergies between subsurface technologies, new urban maintenance regimes and scripts for design.