What is 'Tornado Alley'?
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With survivors picking up the pieces of their homes after this week's deadly tornado in Oklahoma, we examine how and why the state is prone to some of the world's largest twisters. We also look at how the city of Detroit is battling to stave off bankruptcy, while an emergency manager has been called in to make huge spending cuts. And we explore a stunning natural phenomenon under threat: mass butterfly migration from the United States and Canada to Mexico.