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. […]