Drinking water sources, purification, and distribution systems are essential to public health, and failures could be catastrophic.

Security of nuclear power plants was definitely on people's minds as the WTC burned and nobody knew where the next plane would hit.

The Bureau of Reclamation announced laconically Sept. 12 that it had stepped up security at Hoover, Glen Canyon, and Grand Coulee dams.

Because of their length, ubiquity, and remoteness, pipelines can be nearly impossible to defend.

Following the Sept. 11, 2001, attacks on the World Trade Center and the Pentagon, many experts have become more concerned that terrorists may maliciously spread biological agents such as anthrax or smallpox.

U.S. chemical plants are vulnerable to acts of terrorism.

While petrochemical plants get the most attention, statistics from the Chemical Safety Board suggest that media overlook three quite common and widespread hazards: chlorine, ammonia, and propane

Various nations have developed and produced chemical weapons -- substances whose main use is to harm people -- such as nerve gas or mustard gas.

There are places along the Mississippi River (e.g. New Orleans) where many people live in areas that would normally be flooded, were it not for levees or other flood control works.

The WTC victims were killed not just by terrorists, but by the buildings themselves.