You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

Category: U.S. GEOGRAPHY 
Clue: It's 277 miles long, it's up to 18 miles wide, it's 6 million years old & at a given time temperatures within it can vary by 25 degrees
the grand canyon