CBOE shutdown: Options exchange opens again (MoneyWatch) Another day, another trading debacle. All morning long, traders sat idle on the floor of the the Chicago Board Options Exchange, the country's largest options exchange, as it failed to open Thursday due to a software problem. Until noon, trading was frozen in the two most frequently traded options -- the S&P 500 and the VIX, which measures volatility. Investors use such options to hedge their positions on the stock market and other investments. All systems were up and working by noon, said Gail Osten, a spokeswoman for the CBOE. She said the exchange had ruled out hacking as a cause, but said the definitive cause had yet to be found. The shutdown is the second problem for traders and investors this week. On Tuesday a fake tweet prompted a flash crash in stocks, sending the Dow Jones industrial average plunging nearly 150 points before quickly recovering. Other options exchanges remained open during the outage, but none do the same volume of business as the CBOE. A CBOE representative did not return a call for comment.