Developed underground spaces include many tunnels, pipelines, basements, and underground parking garages that quietly serve their cities. These unseen and unnoticed assets may also present excellent opportunities for terrorists. Explosive, flammable, or toxic materials could be brought surreptitiously into the city, placed there, and detonated, largely employing the underground environment alone. Awareness should be the first step in limiting this vulnerability because it can point to the need for surveillance, prevention, and detection of potentially harmful activities in these spaces, thereby limiting exposures. However, several particular concerns that require broader responses would still remain.

Representative Vulnerabilities

Many of our major cities have grown up around railroad lines. Over time, however, the need to separate the railroad’s activities from the evolving city became apparent. As a result, urban railroad lines can be found today in tunnels or along narrow or depressed rights-of-way. Thus they are largely out of sight. Meanwhile, railroads routinely carry all kinds of freight—including toxic chemicals, petroleum products, agricultural supplies, and other materials—that could serve the purposes of terrorists. The U.S. Conference of Mayors has expressed concern about this situation (USCOM, 2001). The risk is greatest for sites above or adjacent to the railroad—such as a stadium or concert hall—that are regularly occupied by great numbers of people.

Some major cities, which have grown up adjacent to large bodies of water, are especially vulnerable to the rapid flooding of their tunnels. Where those tunnels are used for passenger railroad or transit services, significant loss of life could result.

Every city utilizes sewers buried under its streets to convey wastewater and storm water to remote sites for treatment and safe disposal. These sewers typically do not flow full—rather, the water is conveyed by gravity in open-channel flow. Thus, should a volatile liquid be dumped into such a sewer and allowed to flow through it and mix with the air present, an explosive mixture could result. If ignited, a section of sewer might then erupt violently, lifting the street, damaging buildings and nearby tunnels for other utilities, and killing or injuring people.

Underground parking for large urban buildings is the rule rather than the exception today; for one thing, development approvals typically require the availability of off-street parking. But as we learned in the 1993 bombing at the World Trade Center, these under-building parking areas are also desirable locations for terrorist attack. A well-placed bomb could cause much damage to the building’s

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