A bear market is generally defined as a prolonged drop in stock prices, and it occurs when a broad-market index -- such as the S&P 500 -- falls more than 20% from its peak.

Currently, the S&P 500, the Nasdaq, and the Dow Jones Industrial Average are all in bear market territory. That can be unnerving, especially if you have money tied up in the stock market or are thinking about investing.

While there's nothing you can do to prevent a bear market, there is one type of investment that is almost guaranteed to survive even the worst periods of volatility -- the S&P 500 ETF.

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Source Fool.com