Global stock markets are on their shakiest footing in years.

Investors are fleeing stocks and running to safe-havens like bonds and gold, driven by concerns about economic growth and the effectiveness of central banks' policies.

At the same time, tumbling energy prices are upending the economies of oil-producing countries, further slicing into global economic growth.

Only six weeks ago cheap oil prices were still expected to cushion the global economy, and the Federal Reserve's decision in December to raise interest rates for the first time since the end of the financial crisis in 2008 was widely seen as a vote of confidence in the world's largest economy.

In addition to the fall in U.S. stock markets, major stock indexes worldwide have also been hit hard, despite efforts by the Bank of Japan and the European Central Bank to spur growth through lower interest rates.

Large institutions and sovereign wealth funds, who borrowed in euro and yen, have been selling riskier assets, and are now buying back those currencies, undermining central bank efforts.

With Thursday's decline, the S&P 500 stock index has lost 10.5 percent so far in 2016, its worst start to a year in history, according to Bespoke Investment Group, an investment advisory in Harrison, New York. The 10-year note's yield has fallen to 1.63 percent, its lowest closing level since May 2013.

Here are some of the chief issues weighing on the market now.

What Is The Biggest Reason For The Sell-Off?

The slump in equity prices which began late last year has deepened as banks grapple with negative interest rates in parts of Europe and Japan and the flattening of the U.S. Treasury yield curve.