If the US manages to build a bunker-buster powerful enough to knock out Iran's buried nuclear facilities, it can thank some ordnance geeks in Bavaria for helping get the job done.
Here's the story. The American-built FMU-159/B Hard Target Smart Fuze should have been a key part of US bunker-busting arsenal. But it didn't work. Or rather, it "experienced Engineering and Manufacturing Development qualification problems resulting in termination of the program."
...and you want to get the right one. Secondly, you don't want your bomb to overpenetrate, disappearing into the floor and burrowing fifty feet into the ground beneath the bunker before exploding with a faint thump.
The HTSF was supposed to have several modes, capable of counting the number of 'voids' or levels it passed through as well as working on time delay. It was to equip the heavyweight GBU-28 bomb as well as the AGM-86D cruise missile and a host of other weapons. Unfortunately, the aforementioned problems meant that it was not available, so the Pentagon had to fall back on simple FMU-143time-delay fuzes.
Based on the success of the FCT effort, DTRA initiated a Product Improvement Program to package PIMPF for use with the Conventional Air-launched Cruise Missile (CALCM) in order to achieve requirements for standoff defeat of hard and deeply buried targets and to address deficiencies with the current CALCM fuze. This PIMPF PIP (now retitiled as the Void Sensing Fuze Product Improvement Program) will integrate, qualify, test, and deliver the repackaged PIMPF for retrofit into the CALCM weapon system and set the stage for follow-on efforts to address smart fuze requirements for direct attack weapons."
It turns out that the TDW, makers of PIMPF, have been offering it to the US since 2002, as in this presentation entitled "The German Hard Target Fuze is ready." After five years of development, the US version was clearly not ready, and Uncle Sam was forced to buy the foreign product.