The tunnel bomb, a deadly modern riff on an ancient tactic, is emerging as a potent new weapon. Several dozen have been detonated in Syria, while ISIS used them to take the Iraqi city of Ramadi, according to Pentagon officials and documents.

The concept is simple: dig a tunnel long enough to reach under your target, emplace explosives, and hit the detonator. Altogether, at least 45 such bombs have exploded in the past two years in the two countries, according to JIEDDO, the Pentagon organization that seeks ways to defeat improvised explosive devices, or IEDs. Most have been in Syria, but U.S. officials say ISIS is building “a network of bunkers, trenches and tunnels” in Iraq.

“This below the surface attack is particularly destructive to buildings and is appearing increasingly in Afghanistan, Iraq and Syria,” a recent JIEDDO briefing says.

Tunnels have been used for some time by Hezbollah and Hamas in Gaza, generally as passages to smuggle weapons and launch dismounted attacks against Israel. Now their use is spreading, and extending to direct attacks.

In general, tunnel bombs are being used against military checkpoints, buildings and other protected facilities. Short tunnels can be dug in less than 30 days, while longer ones — up to a mile long — take as many as nine months, according to JIEDDO.

“The use of tunnels for IEDs and other purposes will continue to provide a low risk strategic advantage to extremist organizations and therefore requires continued development efforts and fielding of effective mitigation techniques,” JIEDDO says.

Since this type of attack is so effective and destructive, ISIS frequently posts videos of the explosions on YouTube and propaganda websites. The videos show buildings collapsing as massive plumes of smoke and debris fly hundreds of feet into the air.

“As part of an information operations campaign, these attacks are documented and widely proliferated via social media which increases the likelihood of migration to other conflict areas or adoption by other extremist organizations on a worldwide basis,” JIEDDO says.

Iraq & Syria

In Syria, rebels have used tunnels bombs to attack government forces under the control of Bashar al-Assad. Many of these tunnels were dug with hand tools to avoid detection.

In Iraq, ISIS used tunnel attacks to devastating effect in their assault on Ramadi. On March 11, ISIS forces detonated a tunnel bomb under an Iraqi army headquarters, killing an estimated 22 people. The blast consumed seven tons of explosives in an 800-foot long tunnel that took two months to dig, according to the JIEDDO briefing. On March 15, a second tunnel bomb was used to attack Iraqi Security Forces. The city fell two months later.

Beyond bombs, ISIS is believed to be using tunnels to move weapons and avoid detection by American and ally fighter jets and drones. (ISIS may even be exploiting Saddam Hussein’s own tunnel network, which is thought to stretch for 60 miles between palaces, military strongholds, and houses. During the U.S. invasion in 2003, Saddam’s forces used these tunnels to move weapons and as hideouts.)

To find these subterranean passageways, JIEDDO has been seeking help from the scientific community and the oil and gas industry, both of which use specialized equipment and seismic devices to see underground. Some of this technology can be adapted for military use, Col. Timothy Frambes, JIEDDO’s director of strategy, plans and policy, said in an interview Monday. “We’re just trying to figure out what’s the quickest, best technological solution that we can help provide the most complete situational awareness picture of the operating space,” Frambes said.

The work also builds on JIEDDO’s decade-long effort to develop aircraft- and vehicle-mounted sensors that can detect bombs buried in roadways. “The enemy knows that,” Frambes said. “So he has found a way to go subterranean in order to deliver either an explosive charge or just to transit a line of communication.”

The Pentagon is also hoping to learn lessons from Israel, which has sought ways to counter Hamas tunnels in Gaza. That notion is backed by Sen. Kelly Ayotte, R-N.H., who introduced an amendment to the 2016 defense authorization bill that calls for the U.S. to work with Israel on the tunnel problem. The legislation would allow the U.S.“to carry out research, development, test and evaluation on a joint basis with Israel to establish anti-tunneling defense capabilities to detect, map and neutralize underground tunnels into and directed at the territory of Israel.”

A Senate aide said such jointly developed technology could protect battlefield bases and embassies — or even the U.S. frontier. Anti-tunnel work at a site in northern Israel has similar topography to the U.S.-Mexico border, the staffer said.

“Whether it’s criminals smuggling people and drugs into the U.S. under our southern border, or terrorists sneaking into Israel to conduct attacks, tunnels present a serious national security threat to our two countries,” Ayotte said in a statement.

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