Was thinking of the Isis tunnel bombs -
https://www.defenseone.com/threats/2015/06/isis-using-tunnel-bombs-iraq/114730/
Updating an ancient tactic, Islamic State militants — as well as rebels in Syria — are digging virtually undetectable tunnels, then planting bombs to blow up buildings and other targets.
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.
