CHEYENNE MOUNTAIN: Buried inside a mountain beneath 2,000 feet of rock, a top-secret watch post scans the skies around the clock to make sure America is never again caught off guard by an attack like those of September 11.

Designed as a Cold War sentinel against Soviet bombers, Cheyenne Mountain Operations Center near Colorado Springs, Colorado, and the military units it supported were unprepared for the 2001 attacks.

With their eyes trained on threats from abroad, those units failed to stop - or even mitigate - the first attack on US soil by a non-state, foreign enemy which slipped in undetected. But since Sept 11, a massive security overhaul has made Cheyenne Mountain together with the nearby headquarters of NORAD and the new US Northern Command (NORTHCOM) the nerve centre of the military's anti-terrorism network, working hand-in-glove to thwart attacks on North America.

"Before 9/11, we had a very limited internal focus," said Major Charles Thinger, who heads the aerospace warning centre at Cheyenne Mountain. "The morning of Sept 11, there were over 3,000 aircraft flying over the continental United States and NORAD could track less than 20 per cent of them," he said, referring to the US-Canadian North American Aerospace Defence Command in charge of detecting and countering airborne threats.

Today, soldiers inside "the mountain" monitor a dizzying array of screens featuring the real-time flight path of every commercial plane over the United States, as well as constantly updated maps, charts and diagrams channelling information which could presage an attack.

The Cheyenne Mountain centre is a unique military post that is staffed by mixed crews from the US Army, Navy, Air Force, Marines and Canadian armed forces. "There were no procedures in place to mitigate the kind of threat we saw on that day," Navy Commander Mike White said at the joint headquarters in Colorado Springs.

He said that had now changed, thanks to expanded air patrols, radar coverage, information sharing and intelligence coordination - as well as a determination to shoot down civilian aircraft if necessary.

DEFENDING THE US: NORTHCOM, the part of the US military in charge of protecting US land, sea and airspace, was founded only after the suicide hijackings and became fully operational two years later on Sept 11, 2003.