Rather, the system takes advantage of the fact that the earth underneath Fort Knox (like many places across the country) maintains a more or less constant temperature of 57 degrees year around.

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Inside the greenhouse, lines of bright sodium lights shine down on the growing tomato plants, and the temperature remains a constant, warm 23C -- even in the depths of winter when outside temperatures drop well below freezing.

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The entire process takes place in near-freezing conditions and such plants are usually built with large amounts of flammable foam insulation to maintain a constant temperature.

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Besides being sited amid a beautiful setting with a view of the stage and the sound of rushing water, the spring also remains a constant 106.7 degrees Fahrenheit because the staff is always checking the temperature and adjusting the amount of water entering the spring.

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