Looks like the concrete overburden at Natanz may not be all its cracked up to be.

About a year ago, I wrote that I thought the Iranian centrifuge halls at Natanz, although buried, were not likely to survive strikes by conventional, penetrating munitions.

Leaked DIA estimates in 1999 suggest current penetrating munitions can hold at risk facilities with up to 6 m (or about 20 feet) of concrete overburden. Although the halls at Natanz are buried 10-20 meters underground, much of that is rock or soil.

Well, apparently the Iranians are starting to wonder themselves, because commercial satellite images obtained by our friends at ISIS show them tunneling like crazy near Natanz. Sez David Albright and Paul Brannan:

Iran may be constructing a similar facility near Natanz, fearing that the underground halls at Natanz are vulnerable to destruction by military attack. Such a tunnel facility inside a mountain would offer excellent protection from an aerial attack. This new facility would be ideal for safely storing nuclear items, including centrifuge manufacturing and assembly equipment, centrifuge components, natural uranium, and low enriched uranium.

WaPo’s Joby Warrick has more.