New York: Osama bin Laden's killing in a covert American operation in his Abbottabad compound in Pakistan on May 2 was the culmination of months of meticulous planning during which the US explored options like tunneling in and an attack by B-2 Spirit bombers.

Some of the possible courses of action included cooperating with the Pakistani military, which US President Barrack Obama struck down as "there was a real lack of confidence that the Pakistanis could keep this secret for more than a nano second."

One of the options entailed flying helicopters to a spot outside Abbottabad and letting the US Special Forces team sneak into the city on foot.

But, it was felt that the risk of detection was high and the navy SEALs would be tired by a long run to the compound, the New Yorker said in its latest issue giving new insights into what happened in the run up to the May 2 operation in Abbottabad that killed bin laden.

The planners, who started exploring the options in late 2010, had even contemplated tunnelling in and also suspected the possibility of the now dead al-Qaeda chief tunneling out.

"But images provided by the National Geospatial Intelligence Agency showed that there was standing water in the vicinity, suggesting that the compound sat in a flood basin.

"The water table was probably just below the surface, making tunnels highly unlikely. Eventually, the planners agreed that it made the most sense to fly directly into the compound," the report said.

However, Robert Gates, the then Secretary of Defence, was one of the most outspoken opponents of a helicopter assault.

He and General James Cartwright, the then vice-chairman of the Joint Chiefs, favoured an airstrike by B-2 Spirit bombers as that option would avoid the risk of having American boots on the ground in Pakistan.

But the Air Force calculated that a payload of 32 smart bombs, each weighing 2,000 pounds, would be required to penetrate 30 feet below ground, insuring that any bunkers would collapse.

"That much ordnance going off would be the equivalent of an earthquake," Cartwright told the magazine.

The prospect of flattening a Pakistani city made Obama pause. He shelved the B-2 option.