The United States has decided to halt development of a nuclear "bunker buster" bomb. Work will continue on "penetrator" bombs using conventional explosives. No one had ever tried to use nuclear explosives in a penetrator type bomb before. Penetrators rely on specially designed bomb casings, and a very hard front of the case (the part that hits the ground first), to literally burrow though the ground, and concrete, and explode inside an enemy bunker. But there are physical limitations on how much earth or concrete these bombs can penetrate. Currently, the 2.4 ton U.S. GBU-28 can go through up to a hundred feet of earth (depending on what kind of earth), or twenty feet of concrete, before detonating 675 pounds of explosives (hopefully inside a bunker). Putting a nuclear bomb inside the something like the GBU-28 was intended to get at, or try to get at, some of the very deep (more than a hundred feet down) bunkers found in places like Iran and North Korea. The big question was, how much deeper was safe even from a nuclear warhead? It was known, from hundreds of underground nuclear weapons tests, that a bomb exploding underground caused something of a local earthquake. But it was unknown exactly what effect this would have on North Korean or Iranian bunkers. It was this uncertainly, and the general unpopularity of nuclear weapons (and the unpopular prospect of using them for the first time since 1945) that got the nuclear bunker buster project cancelled. There are other ways to get at underground bunkers, including special attacks on the tunnels that link these bunkers to the outside world. Research will continue in that direction.