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.

The key to surviving a bear market

Even the strongest stocks will likely take a hit in the short term when the market is volatile. That's normal. However, those stocks are far more likely to rebound when the market recovers.

If you're looking for a simple way to invest in as many of these strong stocks as possible, a smart option is an S&P 500 ETF -- such as the Vanguard S&P 500 ETF (VOO 0.96%), iShares Core S&P 500 ETF (IVV 0.97%), or SPDR S&P 500 Trust (SPY 0.94%).

An S&P 500 ETF tracks the S&P 500 index, meaning it includes the same stocks as the index itself. Each fund includes stocks from 500 of the largest companies in the U.S., including household names like Amazon, Apple, and Tesla.

Because the companies within the S&P 500 are some of the biggest and strongest in the world, these stocks are far more likely to survive a bear market. By investing in an S&P 500 ETF, your investments also have a better chance of recovering from volatility.

The S&P 500 also has a fantastic record of rebounding from bear markets. In fact, it's managed to recover from every single downturn it's ever faced. While nobody can say for certain how long this bear market might last, it's extremely likely an S&P 500 ETF will see positive average returns over the long term.

Is now really the right time to invest?

Investing during a bear market can be daunting. After all, continuing to invest as stock prices plunge can feel like throwing your money away. However, there are two good reasons why right now is a fantastic time to buy.

1. Stock prices are lower

Plummeting prices are normally not a great sign for the market, but they do make for a smart buying opportunity. By investing right now, it's possible to snag high-quality stocks for a fraction of the price.

For example, the Vanguard S&P 500 ETF currently costs around $330 per share. Back in early January, though, it cost roughly $436 per share. When you buy now, you're getting the exact same investment, but at a $100+ discount.

2. You can earn significant returns when the market rebounds

The fundamental rule in investing is to "buy low, sell high." In other words, invest when prices are at their lowest, then hold those investments until the market inevitably recovers.

Right now is a fantastic opportunity to "buy low," and because the S&P 500 is almost guaranteed to rebound at some point, you can set yourself up for substantial returns when stock prices bounce back.

One caveat to consider, though, is your financial situation. Avoid investing any money you may need in the next few years, as it could take months or even years for the market to fully recover. If you don't have any emergency savings, it may be best to focus on that goal before you invest.

However, if your finances allow for it, now can be a smart buying opportunity. S&P 500 ETFs aren't for everyone, but they can provide a safer option for investing in the stock market while still maximizing your earnings.