Fiction: In one of the film's more implausible concepts, Earth has adapted to discourage human habitation by freezing over completely, each and every night. To survive, Kitai must find geothermal "hot spots" whenever the sun goes down.
Science: The kind of deep-freeze scenario presented in the film would seem to discourage, with extreme prejudice, all plant and animal life. Still, the meteorological record does feature some pretty wild temperature swings. In 1911, the temperature in Rapid City, S.D., dropped from 55 degrees Fahrenheit to 8 degrees in about 15 minutes. The largest 24-hour temperature change in the United States was recorded in Browning, Mont., in 1916 -- from 44 degrees to minus-56 degrees.