"It’s a tough decision. But we made it in Japan.”

One of the military’s initial option plans, as presented to the White House by the Pentagon this winter, calls for the use of a bunker-buster tactical nuclear weapon, such as the B61-11, against underground nuclear sites. One target is Iran’s main centrifuge plant, at Natanz, nearly two hundred miles south of Tehran. Natanz, which is no longer under I.A.E.A. safeguards, reportedly has underground floor space to hold fifty thousand centrifuges, and laboratories and workspaces buried approximately seventy-five feet beneath the surface. That number of centrifuges could provide enough enriched uranium for about twenty nuclear warheads a year. (Iran has acknowledged that it initially kept the existence of its enrichment program hidden from I.A.E.A. inspectors, but claims that none of its current activity is barred by the Non-Proliferation Treaty.) The elimination of Natanz would be a major setback for Iran’s nuclear ambitions, but the conventional weapons in the American arsenal could not insure the destruction of facilities under seventy-five feet of earth and rock, especially if they are reinforced with concrete.

(...)

The attention given to the nuclear option has created serious misgivings inside the offices of the Joint Chiefs of Staff, he added, and some officers have talked about resigning. Late this winter, the Joint Chiefs of Staff sought to remove the nuclear option from the evolving war plans for Iran—without success, the former intelligence official said. "The White House said, 'Why are you challenging this? The option came from you.'"

Seymour Hersh, The Iran Plans