 Minimum metal mines are extremely difficult to detect using conventional metal mine detectors and usually require modern techniques, such as robotic "Multi Period Sensing" (MPS) equipment, to identify, but it is still extremely difficult to find non-metallic mines.These techniques are usually restricted to well-funded international mine clearing organizations and major militaries, making minimum metal mines especially pernicious where they are encountered.Early examples included the German Glasmine 43, Schu-mine 42 and Topfmine used during World War II.