When U.S. and Iraqi troops invade the Islamic State group's stronghold in Mosul, a big part of the battle may occur underground.

The massive military operation, expected to begin in October, will be complicated by a network of tunnels beneath the city. These labyrinths will offer a key tactical advantage to the estimated 4,000 to 10,000 ISIS fighters who are entrenched in Mosul.

"Daesh are very keen to build tunneling networks because it helps them to maintain security and preserve their lives a little bit longer whenever clearing operations begin," said Air Force Col. John Dorrian, a top spokesman for U.S. troops in Iraq. Daesh is another name for ISIS. "In a city the size of Mosul," he added, "that's going to, you know, slow things down and make it a complicated task." 

Tunnels pose a vexing problem for the U.S. military, which lacks reliable technology to detect and map the subterranean pathways. They'll make small-unit operations extremely difficult.

If, for instance, the passageways are built with zigzags, small-arms fire will be ineffective. They could be booby trapped to kill intruders. Oxygen can run short. Traditional communications devices might not function well. Medical evacuation will be slowed if walls collapse or if small exit points are covered by enemy fire.

Coalition forces found extensive tunnel networks under Ramadi and Sinjar, key battlefields in Iraq that were liberated within the past year. In Syria, American-backed rebels found an "intricate tunnel complex" in the town of Manbij, U.S. officials said in June.

ISIS fighters are known to reinforce the walls within these tunnels and equip them with electricity, lighting and ventilation systems. They allow militants to hide from airstrikes, move without detection from overhead surveillance and escape from close-quarters combat.

Confronting tunnels is a concern for the Pentagon. Last year, an initiative seeking new ideas for the Defense Department’s Combating Terrorism Technical Support Office identified "detection of underground voids (tactical tunnels)" as a top priority.

Congress included $40 million in its 2016 spending bill to fund anti-tunnel technology, an joint effort between the U.S. and Israel. The Israeli military has struggled to counter elaborate tunnel structures built by Hamas.

At the Army’s Fort Hood, an extensive tunnel network offers soldiers a place to train for underground combat operations that demand unique tactics and pose non-traditional problems.

The military also is researching ways to use robots to explore and map underground tunnels, including some shaped like snakes and small hovercrafts that collect imagery. The Defense Department’s 2016 "Rapid Innovation Fund" seeks industry help with developing a cell phone-size device capable of voice communication and geolocation within a tunnel, according to budget documents published in March.

Another request for new technology seeks a "temperature measurement system" for surveillance aircraft that could detect tunnels by finding small variations between the temperatures of ground soil over a tunnel compared to the surrounding area.

For now, there’s no way for U.S. and Iraqi troops to know what kind of tunnel system ISIS might have in Mosul.

"They have had two years there in order to prepare," Dorrian said in during a recent press briefing. "I think this is one of those cases where, from the air, you can see a lot and there is probably a lot that you cannot see.

"We're going to anticipate a very tough fight. We're going to anticipate that they are going to do those kinds of things because we have seen them elsewhere," he added. "But the Iraqis have had a good track record of success, especially in the last year, and they are beginning to get a very good understanding of what they can expect" from ISIS.

Andrew Tilghman is the executive editor for Military Times. He is a former Military Times Pentagon reporter and served as a Middle East correspondent for the Stars and Stripes. Before covering the military, he worked as a reporter for the Houston Chronicle in Texas, the Albany Times Union in New York and The Associated Press in Milwaukee.

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