US airstrikes on Iran’s Fordow nuclear facility appear to have left at least six large craters, pointing to the use of bunker busting bombs, a CNN analysis of satellite imagery found.

The images captured by satellite company Maxar showed six visible, separate impact craters in two nearby locations at Fordow — a highly secretive site buried deep in the mountains. The craters can be seen along a ridge running over the underground complex.

Earlier Sunday, a US official confirmed to CNN the US had used B-2 bomber planes to drop more than a dozen massive “bunker buster” bombs as part of the attack on Fordow and two other nuclear sites. These weapons can penetrate deep underground before detonating and are seen as essential for targeting the facility, much of which is buried beneath the ground.

Satellite imagery also showed significant changes to the color of the mountainside where the site is housed, indicating a vast area was covered with a layer of grey ash in the aftermath of the strikes.

As of yet, the level of damage to the underground facility is unclear.