Observation: Underground space is a valuable but decidedly nonrenewable resource.
Conclusion 10. Underground space can enhance urban sustainability only if the underground is thoroughly understood and if underground use and reuse and the protection of the natural and built environments are incorporated into long-term total urban infrastructure system planning.
a. Institute planning of all underground space as part of an evolving urban system to be carefully engineered or preserved for optimal long-term use and regional sustainability.
b. Establish reasonably intensive groundwater, soil, and infrastructure monitoring practices to track the health of the underground urban environment according to the general geologic conditions and use. Use data generated from a range of environments and situations to inform urban planning in other areas.
It is easy to look at a photograph of a city and envision a three-dimensional model of its surface structures, skyscrapers, and raised highways. This report challenges many urban planners, designers, engineers, researchers, contractors, and infrastructure operators to include the subsurface in this three-dimensional model, and to coherently link infrastructure between the surface and subsurface. Just as there is only so much surface area in a given city, there is only so much usable underground volume beneath the surface. However, unlike infrastructure on the surface, underground infrastructure cannot be easily removed or rebuilt when its useful life ends. Once subsurface geologic materials are removed and infrastructure elements or waste are put in their place, the subsurface cannot be restored to its original state and possibly may not be used for other purposes. For this reason, urban sustainability is dependent on thorough understanding of the underground and how best to plan for the use, reuse, and protection of underground resources—whether referring to natural energy or material resources, or to the underground space itself.
People have exploited underground space and resources for thousands of years to advance and protect survival, economic prospects, mythological culture, and spiritual growth. These endeavors involved high risks offset by the belief that the benefits of the underground exceeded the dangers—long before there was detailed understanding of the underground environment or sophisticated tools with which to explore it. However, early successes and failures in the underground helped build the substantial knowledge base that exists today throughout the world. The challenge now is to create a comparable legacy to sustain the