The U.S. Air Force began designing the GBU-57/B, also called a bunker buster, in 2004 specifically to attack nuclear facilities deep beneath mountains. Other American bunker busters, like the BLU-109 and the GBU-28, aren't able to penetrate the thick layers of soil, rock and concrete protecting Fordo.

As U.S. Considers Using Bunker-Buster Bombs, Here’s What It Takes to Hit Iran’s Deepest Nuclear Site
The New York Times