Techniques for collecting and removing marine (or riverine) debris include the use of debris skimmer boats "(pictured)". Devices such as these can be used where floating debris presents a danger to navigation. For example, the US Army Corps of Engineers removes 90 tons of "drifting material" from San Francisco Bay every month. The Corps has been doing this work since 1942, when a seaplane carrying Admiral Chester W. Nimitz collided with a piece of floating debris and sank, costing the life of its pilot. The Ocean cleanup has also created a vessel for cleaning up riverine debris, called Interceptor. Once debris becomes "beach litter", collection by hand and specialized beach-cleaning machines are used to gather the debris.