Gravel mines, also called Button mines were small U.S. made air-dropped anti-personnel mines. They were used extensively during the Vietnam War as part of the McNamara Line. They were also used as a rapid-deployment area denial expedient, to provide a barrier during combat search and rescue (CSAR) operations between downed pilots or other endangered units and infantry threats. 37 million gravel mines were produced between 1967 and 1968, though mines were produced into 1970.