Bunker buster bombs use advanced science to function, as they must literally penetrate deep into a target location before detonating. The one currently in use by the United States is the GBU-57/A/B Massive Ordnance Penetrator (MOP) with the BLU-127. It was used for the first time in the attack against Iran in Operation Midnight Hammer, and it appears to have worked as designed. The GBU-57 is massive, weighing in at 30,000 pounds., making it heavier than the legendary so-called “Mother of All Bombs.”

The weight is required for penetration, but it’s hardly the only aspect of the bomb’s design that functions for that purpose. The bomb measures 20.5 feet in length and carries an explosive warhead weighing 5,300 pounds. That makes the bomb’s primary heavy component its hardened penetrator, which is what enables the ordnance to penetrate over 200 feet of reinforced concrete or rock. It is guided via GPS and is 10 times more powerful than its predecessor, the BLU-109.

As it happens, while the GBU-57 functioned as designed in its only use in combat, the U.S. Air Force is already looking for a replacement. One of the main reasons for this is the delivery system, as currently, only the B-52 Stratofortress and B-2 Spirit stealth bomber can carry and deploy the MOP. With the B-21 Raider in development and the the Air Force eyeing to replace these legacy strategic bombers, it wants to continue being able to devastate hardened targets. Until that program comes to fruition, the GBU-57 will likely continue to operate for the Air Force.

While almost all bunker buster bombs use conventional warheads, not all of them do. The United States uses the B-61 variable-yield thermonuclear gravity bomb as its primary nuclear bomb, and one model, the B-61 Mod 11, is a bunker buster variant. If you think a bunker can survive a surface detonation from a nuke, it’s unlikely, though some can, depending on depth and other factors, which is why the B-61 Mod 11 was developed. Some might think that adding a nuclear warhead to a bunker buster is overkill, but there’s a good reason for its creation.

A conventional bunker buster is designed to penetrate and destroy a single target. A nuclear model, on the other hand, is designed to take out an entire underground complex of bunkers and support structures. As you likely know, these weapons have never been used in combat — the only nuclear devices used in warfare came in World War II during the bombings of Hiroshima and Nagasaki, Japan. Despite this, around 50 B-61 Mod 11 nuclear bunker busters remain in the U.S.’ inventory.

Instead of direct penetration, the B-61 Mod 11 is deployed in what is called a laydown mode. It uses a Kevlar parachute to slow its descent before landing on the ground, which also gives the aircraft that dropped it time to get to a safe distance. In ground-burst or laydown modes, the bomb’s variable yield of between 0.3 and up to 340 kilotons suggests little beneath or around the bomb would survive detonation, including hardened bunkers buried as deep as 1,000 feet beneath ground zero.