In typical mooring situations, vessel security is most vulnerable to attacks from beneath the water's surface. Even the most extensive foot and vehicular patrols, access control systems, and land security measures cannot prevent waterborne attacks on vessels or the underwater portions of piers, locks, power lines, and installations. Underwater terrorist or sabatour bomb attacks have many advantages. A clandestine approach is enhanced, multiple points of entry and egress are available, pursuit by security personnel is difficult, and explosive devices can be rigged or cached prior to use and employed when needed. Disadvantages include the need for increased training, support, and coordination; limitations imposed by underwater conditions; and time and depth restraints. The method of attack, equipment and explosive devices used, and logistics are as varied as the terrorist groups and their motives. Limpet mines and special-attack charges vary in weight and size and method of delivery. Self-contained or closed-circuit underwater breathing apparati may be used. Underwater delivery vehicles can be highly sophisticated or rudimentary. In developing an underwater security plan, managers must evaluate the potential terrorist threat taking into account criticality, accessibility, recuperability, vulnerability, effect, and recognizability. An encompassing seaport and vessel security plan is the only solution to the menace of waterborne terrorism. Photograph.