A clear, hot July day in 2010 seemed like the perfect opportunity for a day trip to Donna Cooper. With her daughter Gina and visiting friend Jenny, Cooper set out for Scotty's Castle, a tourist museum in Death Valley. They packed what they thought would be enough food and water for the day, along with "Nell," Cooper's seemingly trusty GPS device. An uninvited companion was the weather -- the temperature would reach 121 degrees in Death Valley that day. Watch the full story on "20/20: Highways From Hell" Friday at 10 ET

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