The U.S. Army is now concerned about anti-helicopter mines:
Bulgaria, which seems to have developed these devices as far as as the late 1990s, offers several mines such as the AHM-200, a 200-pound device which looks like a mortar tube mounted on a tripod. The mine, which is emplaced on the surface rather than buried in the dirt, has an acoustic sensor which arms the weapon when it picks up the sound of the helicopter as far away as 1,500 feet. At a range of 500 feet, a Doppler radar tracks the target. When the helicopter gets within 300 feet, the mine detonates both an explosively formed projectile and an explosive charge packed with steel balls.
A 2012 Russian news video shows what looks a similar device. A Russian expert in the video claims that anti-helicopter mines were developed because shoulder-fired anti-aircraft missiles are ineffective against helicopters flying lower than 300 feet.
Other nations have also developed anti-helicopter mines. Poland has one, while Austria has developed an infrared-guided version.