Hold that update!
Today the USA finally got to use the GBU-43, now commonly known as the Moab (popularly, the 'Mother of All Bombs'). It was dropped on the Afghanistan’s Nangarhar province, in an area believed to be riddled with ISIS tunnels and bunkers. It is believed to be the largest non-nuclear bomb ever used in combat.
This is indeed still the biggest non-nuclear bomb in the US arsenal, and yes, it has sat dormant for 14 years. This is simply due to the fact that it is not particularly effective for doing most things, but might, we hope, be really good at damaging large networks of underground tunnels... in much the same way as an earthquake is.
The Moab is simply a missile-shaped block of explosives, which is dropped onto the location that you wish to explode. This is somewhat unusual; missiles usually have frangible casings, shaped charges, submunitions, incendiaries, and other add-ons to optimize the damage. Even the fins on the Moab are duds, existing specifically to slow it down, because otherwise the lumbering planes needed to carry the massive bomb (in this case, a MC-130) might not be able to get out of the blast range before impact.
The blast is equal to 11 tones of TNT (to put this in context, the first nuclear bomb equaled 20,000 tons of TNT; the average Tomahawk, if I've done my math correctly, equals about 0.5). This is an effective way of creating a minor, but fairly wide-spreading earthquake, which is good if you want to take out a lot of tunnels.
We don't usually want to do that, but we have about 10 more of these bombs in storage, so we'll probably try again before this decade is over.