With Donald Trump mulling whether to send in American bunker-busting bombs to deal a crushing blow to Iran’s nuclear facilities, doubts remain about just how close Iran is to creating a nuclear bomb.

Israel repeatedly struck Iran’s largest enrichment site at Natanz and at its Isfahan nuclear complex. But the full extent of the damage to the country’s nuclear program—or Iran’s ability to race secretly toward a nuclear weapon—remains unclear.

Much of the world views Iran’s nuclear program with alarm, and experts say its stockpile of highly enriched uranium has grown fast.