<p><strong>Sandbags block the entrance of the New York Stock Exchange in downtown Manhattan as Hurricane Sandy bears down on the city on October 29. </strong></p><p>The superstorm, which is slamming into a western storm and cold air from the Arctic, could endanger up to 50 million people for days, CBS News reported.</p><p>"This is the worst-case scenario," Louis Uccellini, environmental prediction chief for the National Oceanic and Atmospheric Administration, told CBS News.</p><p>Hundreds of thousands have already moved to higher ground, public transport has shut down in three major metropolitan areas—Washington, D.C., Philadelphia, and New York City—and the U.S. stock market suffered its first weather-related closure in 27 years, according to Reuters.</p><p>(See related blog post: "<a href="http://newswatch.nationalgeographic.com/2012/10/26/hurricane-sandy-could-be-one-of-most-destructive-storms-in-many-years/http://newswatch.nationalgeographic.com/2012/10/26/hurricane-sandy-could-be-one-of-most-destructive-storms-in-many-years/">Hurricane Sandy Could Be One of Most Destructive Storms in Many Years</a>.")</p>