The only conventional bomb that could destroy Iran's buried nuclear facilities, the GBU-57, is one of the United States' primary strategic tools in the Israel-Iran war.

This 13-ton bunker-busting warhead, which Israel does not possess, is capable of sinking tens of meters below the surface before exploding. A feat almost unique in the hands of Donald Trump, should he decide to intervene.

Why this bomb?

Although the Israeli army managed to decimate the Iranian military command and numerous facilities in five days,"many doubts remain about the effectiveness of the Israeli attacks against the core of Iran's nuclear program," according to Behnam Ben Taleblu.

According to this expert from the Foundation for the Defense of Democracies, a neo-conservative American research center,"all eyes are on Fordo."

"No damage has been observed" at this uranium enrichment plant, according to the International Atomic Energy Agency (IAEA).

Unlike the Natanz and Isfahan facilities, this one is located deep underground, about 100 meters below ground, out of reach of Israeli bombs.

"Only the United States has the conventional capability" to destroy such a facility, said Mark Schwartz, a U.S. general who served in the Middle East and now a researcher at the Rand Corporation think tank.

This"conventional", i.e. non-nuclear, capability is the GBU-57.

What is its scope?

What makes this American bomb unique is its ability to penetrate rock and concrete.

The GBU-57 was designed to penetrate up to 61 meters underground before exploding, the U.S. military says.

Unlike many missiles or bombs that detonate their payload on impact, these bunker-buster warheads first seek to burrow into the ground and only explode when they reach the underground facility.

These weapons are designed with a very thick, reinforced steel casing  to help penetrate these rock layers, said Masao Dahlgren, a weapons specialist at the CSIS research center in Washington.

This explains its weight: more than 13 tons and 6.6 meters long.

Its effectiveness also lies in its detonator, which is not activated upon impact, but rather detects when the bomb reaches an open cavity and detonates upon entering the bunker, according to Dahlgren.

The design for this bomb was launched in the early 2000s. In 2009, an order for 20 units was placed with Boeing.

How do you cast it?

The American B-2 aircraft are the only ones capable of delivering this bomb.

Some of these stealth strategic bombers were deployed in early May to the US base on Diego Garcia in the Indian Ocean, but by mid-June there was no trace of them, according to an AFP analysis of satellite images from PlanetLabs.

Thanks to their long range, B-2s taking off from the United States"are capable of flying to the Middle East to carry out bombing raids; it's been done before," according to CSIS's Dahlgren.

Each B-2 can carry two GBU-57s.

If they decide to use them,"they won't just drop one (bomb) and that's it, but they'll use several to ensure a 100 percent chance of success," predicted Mark Schwartz.

Israel's air superiority in Iran"reduces the risk" of such an operation, the retired general added.

What consequences would its use have?

Such an intervention would have a high political cost for the United States, estimated Behnam Ben Taleblu. And"it is not the only solution," he added.

In the absence of this US bomb, an Israeli attack on an underground complex like Fordo could involve"attempting to attack the entrances, demolishing whatever they can, cutting off the electricity," this expert listed. This strategy appears to have been implemented at the Natanz plant.

There is also a recent and little-known nuclear facility near Natanz"probably even deeper than Fordo" and "where one wonders what is going on there," he concluded.