Excerpt below:

Wall Street has been on a roller coaster ride. On Tuesday, the market regained close to half of its deep Monday loss, but on Wednesday the market ended down again.

At Thursday's opening bell, it bounced from positive to negative, then headed down sharply.

The "sell-off has now corrected prices back to the level that preceded the passage of the tax bill" in December, said David Kotok, chairman of Cumberland Advisors. Enthusiasm about a corporate tax cut had helped drive the stock market to record levels.

Several factors are contributing to the return of neck-wrenching volatility this week after years of absence in the market. In fact, investors had devised very profitable investment products that allowed them to bet that volatility would remain low. Those bets against market volatility blew up on Monday.

Read the full story at NPR.