The M1, M1A1 and M4 are circular steel cased United States anti-tank blast mines with a distinctive cross shaped pressure spider. The mine was used during the Second World War and a copy produced in Argentina was used during the Falklands War. Another copy is produced in China, there designated as the No. 4 mine. Variants of the mine have also been deployed in Chad and Tunisia.
Downward pressure on the spider presses down on the head of the fuze, breaks a shear pin, and inverts a belleville spring, flipping the striker downwards into a stab sensitive detonator, triggering the mine. The M4 version of the mine has two secondary fuze wells so that anti-handling devices can be fitted.