The New York Stock Exchange is down. All open orders have reportedly been cancelled, and all trading has been suspended.
Officially, there's no word yet on why the exchange is down, but CNBC says an NYSE official has indicated that the exchange is suffering from "a major technical issue."
According to a tweet from Jennifer Bendery of The Huffington Post, the Department of Homeland Security has said that, so far as they know, there is "no indication" of any sort of cyberattack.
The halt came after what The Wall Street Journal describes as a "broad selloff in shares that was spurred by a deepening decline in China's stock market." (Read more about China's financial system troubles here.) It's not clear when the exchange might reopen.
Update: In a Tweet, the NYSE confirms that they are "experiencing a technical issue" and are working to resolve it.
Update 2: Looks like trading will resume at 3:10 p.m. this afternoon.