In 1991, during the first Gulf War, the U.S. military recognized a need for weapons that could pierce underground concrete bunkers and hardened facilities.

In three weeks military researchers fashioned a hardened steel bomb made from an artillery casing that could be dropped from a high altitude, pierce earth and concrete, and detonate inside underground bunkers.

The ability to crack open facilities that previously were resistant to conventional ordnance meant bunker designers had to dig deeper, wrapping important facilities in extra layers of concrete and burying them under mountains.

Few countries are better at protecting their facilities under layers of