Case hardening. Making something which if propelled fast enough (secondary issue) and with a G force resisting detonator (secondary issue) which has sufficient integrity and inertia to penetrate as deeply as possible before exploding. Materials science in making aerodynamic rigid, shock tolerant materials to fling at the ground.

I am sure the materials science aspects have come along since ww2, as has delivery technology, but I'd say how it goes fast, hits accurately and explodes is secondary to making a case survive impact and penetrate.

I would posit shaped charges could be amazing in this, if you could make big ones to send very high energy plasma out. I'm less sure depleted uranium would bring much to the table.

(Not in weapons engineering, happy to be corrected)

According to public information, Eglin steel.

I was guessing either tungsten or depleted uranium, as for APDS, but the bomb's average density is only about 5 g/cc (14 tonnes in 3.1 m³). Length of 6.2 m times 5 tonnes per cubic meter gives a sectional density of 31 tonnes per square meter, which is about 15 meters of dirt. So Newton's impact depth approximation would predict a penetration depth one fourth of the reported 60-meter depth.

I don't know how to resolve the discrepancy. The plane wouldn't fly if the bomb weighed four times as much. Maybe most of the bomb's mass is in a small, dense shaft in the middle of the bomb, which detaches on impact?

It's not that crazy. It's simple physics. Drop a 15 ton metal lawn dart from 50,000 feet and it has a lot of energy.

No real secret sauce, the weapon weighs almost 30,000lbs and most of it is just hardened metal to make it heavy. The warhead is only ~5,300lbs of explosive