The two 23-ton blast doors at the entrance inside the Cheyenne Mountain Complex are made of steel and can take up to 20 seconds to close with the assistance of hydraulics. If the hydraulics were to fail, the military guards stationed in the tunnel can close the doors in 40 seconds. /Staff Sgt. Andrew Lee

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Photo credit: © 001 Images / Alamy / Afripics
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Keywords: air, airforce, cheyenne, colorado, conflict, force, military, station, usa, war