In 1960, during the height of the Cold War, the US Air Force introduced the Titan I, its first series of multi-stage Intercontinental Ballistic Missiles. Along with the Atlas missile program, they became an integral part of the US's nuclear deterrent against the Soviet Union.
The Titan-I and the machinery and crew needed to launch them were housed in massive underground silos connected by thousands of feet of tunnels. Missile bases were built all over the United States, costing the US government millions.
But by 1965, the Titan-I's and their bases were all but abandoned, phased out in favor of newer and more advanced missiles.
A few Titan-I bases still remain intact, buried reminders of some of the most dangerous years of the Cold War.
Urban explorer and photographer Amy Heiden gained access to one of them, once part of Beale Air Force Base, in Chico, California, and documented the sprawling 1960s nuclear base lurking just below the earth's surface.
More of Heiden's work can be seen on her website and blog.
From the surface, the base look unassuming. But much lies below these launch doors.
Over 600,000 cubic yards of earth were removed in the facility's construction, and "32,000 cubic yards of concrete, 300 tons of piping, 90 miles of cables and 1,800 separate supply items" were used for each complex, according to the US Air Force.
Once underground, there's an intricate system of tunnels which connect three missiles silos to each other, as well as to other areas of the base.
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