why the next battlefront could be subterranean Fighting underground: An Israeli soldier crawls through a tunnel built by Palestininans from the Gaza Strip town of Rafah into Egypt which the Israeli army discovered in May, 2002. WASHINGTON: For more than a decade, the United States has targeted insurgents from the sky with increasingly advanced drones, launching air strikes in Iraq, Afghanistan, Pakistan, Yemen and other volatile countries. But the practise may be contributing to a new trend: foreign militaries and insurgents are using tunnels and other underground networks more and more to hide and gain a tactical advantage - and that increases the likelihood that US forces will face them below ground in the future. The US Army just issued a warning about tunnel warfare as part of a new effort seeking high-tech robotics, communications gear and other equipment. Army officials requested industry's help earlier this month, saying "the growing use of tunnels and underground facilities by military and irregular forces to gain a tactical advantage is becoming more sophisticated and increasingly effective, making the likelihood of US Forces encountering military-purposed subterranean structures on future battlefields high." The Army did not identify any specific country in which they expect tunnel warfare will occur, but said the Middle East is full of ancient and modern underground systems that can be used by enemy forces. Examples include Syria, where rebels have used them extensively; Iraq, where they are rumoured to stretch for miles; and Egypt, where the military flooded many of them with sewage earlier this year, before President Mohammed Morsi was removed from power. Tunnel networks stretch well beyond the Middle East, however. In southern Afghanistan, Taliban fighters have used them to hide weapons and to disappear after ambushing US forces. In South Korea, military officials fear their North Korean counterparts have dug a series of deep tunnels that will allow them to launch a fierce invasion of their US-aligned neighbour. And in Mexico, tunnels have been dug underneath the country's border with the United States to smuggle in massive quantities of cocaine and marijuana. Historically, tunnel warfare also played a prominent role in World War II, where U.S. forces fought Japanese troops who were deeply entrenched in bunkers connected by a series of tunnels on Pacific islands like Iwo Jima. US troops used flame throwers, small arms and grenades to root them out, but the Japanese were frequently able to sneak back into areas that had been cleared through tunnels the Americans did not know existed. Army officials said in the notice to industry that it needs not only specialised equipment for underground combat operations, but to have personnel available with specialised training in it. Some of the equipment it says it needs for tunnel warfare comes into play in just about any combat environment - ballistic shields, for example. Other needs are specific to life underground, however. For example, the service is interested in equipment that can map out underground environments, even when a GPS signal is not available, and radios that work underground. The Army also asked for information about breaching gear that can cut or blow holes into new walls, breathing devices for when oxygen is scarce, and gear that will allow soldiers to see in the dark. They could include light sources, thermal imaging devices or items that can create light after being thrown. The Army asked private companies to submit white papers outlining their suggestions, and suggested they would begin hosting demonstrations for the effort beginning in early 2014.