The heavyweight explosive known as a GBU-57A/B Massive Ordnance Penetrator, which only the United States has, is needed by Israel to destroy Iran’s underground nuclear facility at Fordow, weapons experts have said, reports the New York Post.

The Fordow nuclear facility is located in a mountain, about 90 meters deep, and its destruction requires bombs that can destroy objects at great depths.

“The United States controls the bomber and the bomb,” John Spencer, chair of urban warfare studies at the Modern War Institute at West Point military academy, told The Post. “It would be an American plane and an American munition.”

The bomb digs a certain distance deep into the ground, then explodes, which enables it to reach underground structures.

The missile cost over $500 million for the US Army to develop, and it weighs 15 tons. Its huge weight means it can only be delivered with a B-2 Spirit stealth bomber—a jet that Israel’s air force does not possess.

According to a 2013 article in The Wall Street Journal, at that time 20 of the bombs had been manufactured for the US military. But Washington has not transferred them to allies, including Israel.

On June 13, Israel attacked Iran, and this was followed by retaliatory measures by Iran. Military actions between the two countries are continuing to this day.