DENVER - The U.S. military is moving its airspace surveillance headquarters out of a Cold War-era bunker complex built inside a mountain in the western U.S. state of Colorado.

The North American Air Defense Command, known as NORAD, will move day-to-day operations to a typical office building on a nearby Air Force base.

Officials Friday said threats have changed since the United States was engaged in a Cold War with the Soviet Union.

They say the shift will make operations more efficient and economical. And they stress the mountain compound is not closing, and will be kept as a back-up.

More than 40 years ago, the military constructed the Cheyenne Mountain Operations Center, hundreds of meters beneath the mountain.

The 15 buildings that make up the complex are mounted on 1,300 shock absorbers to protect people and equipment from missile strikes.

The government has spent $700-million upgrading systems at the center since the September 11, 2001, terrorist attacks on the United States.