COLORADO SPRINGS, COLO. (AP) -- A Cold War-era bunker deep inside a Colorado mountain has lost its most famous tenant, but it's still home to some key national defense units.

The Missile Warning Center inside Cheyenne Mountain just got a $2.9 million renovation with upgraded computers and ergonomic work stations.

The center monitors for nuclear detonations, missiles and objects re-entering the atmosphere from space.

Cheyenne Mountain in Colorado Springs was excavated in the 1960s to withstand attack by the Soviet Union. It's best known as the former command center for the North American Aerospace Defense

Command, which defends the U.S. and Canadian skies.

NORAD moved its command center to nearby Peterson Air Force Base starting in 2006, citing cheaper operating costs and a reduced threat of attack.

Cheyenne Mountain remains a backup for NORAD.

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