GOING UNDERGROUND: THE U.S. GOVERNMENT'S HUNT FOR ENEMY TUNNELS.

• Author: Harper, Jon

In April of last year, the U.S. military dropped the most powerful non-nuclear bomb ever used in combat on a tunnel complex in Afghanistan's Nangarhar province. The airstrike targeted the Islamic State's Khorasan branch. The use of the GBU-43/B Massive Ordnance Air Blast weapon, the so-called "Mother of All Bombs," highlighted the growing threat posed by adversaries' underground structures.

Gen. John Nicholson, commander of U.S. Forces-Afghanistan, cited the challenge of dealing with subterranean targets to justify the use of the bomb, also known as the MOAB.

"As ISIS-K's losses have mounted, they are using [improvised explosive devices], bunkers and tunnels to thicken their defense," he said in a statement released after the attack. "This is the right munition to reduce these obstacles and maintain the momentum of our offensive."

In other locations such as the Iraqi city of Mosul, ISIS built an extensive network of tunnels to move fighters, launch attacks and hide from coalition aircraft.

Meanwhile, drug cartels are using tunnels to smuggle contraband into the United States from Mexico.

Sixty-seven of them were discovered from fiscal years 2011 through 2016, according to the Government Accountability Office.

"In a lot of cases they start [digging a tunnel] in a building on one side of the border and they end in a building on the other side of the border," said Mark Kaczmarek, a program manager at the Department of Homeland Security's science and technology directorate.

Advanced sensing technology is needed to find them. "It's not like you just have a big hole in the ground on one side or the other that you can just see a pile of dirt showing up all of a sudden," he said in an interview with National Defense.

Additionally, hostile regimes such as North Korea are believed to be hiding WMD technology and other weaponry in underground facilities that the U.S. military might need to locate.

To tackle these challenges, the United States government is developing new systems to find and investigate subterranean complexes.

"There's a variety of technologies that you can use to detect voids in the ground... but geology plays a major role in that process," Kaczmarek said.

"Certain types of geology will absorb certain types of radiation whether it's acoustic, whether it's electromagnetic, RF energy, etc.," he explained. "You may find that one technology that you field may be suited for a particular area and its geology [but] may not be suited for another area and its geology. So if you have a place that's highly rocky versus highly sandy versus clay--all that plays into how well any given technology works in trying to detect tunnels."

The depth and size of a tunnel and the types of materials inside it are also important factors, he...