Over the past year and a half, the stock market has experienced one of the greatest growth streaks in history. The S&P 500 is up nearly 100% since March 2020, and investors have seen their portfolios skyrocket during that time.

Some experts believe, however, that it's only a matter of time before prices fall. Market downturns may be intimidating, but they're normal. In fact, it's healthy for the market to experience corrections every so often, because stock prices can't continue climbing forever.

Nobody knows for sure whether a crash is on the horizon, or, if it does happen, how significant it will be. However, if the market does take a turn for the worse, there are a few exchange-traded funds (ETFs) I'll be buying.

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1. Vanguard S&P 500 ETF (VOO)

It may seem counterintuitive to buy when the market is down, but crashes can be a fantastic opportunity to invest when prices are lower. ETFs trade like stocks, so when the market is down, their share prices are typically lower as well.

One ETF I'm planning to invest in heavily if the market crashes is the Vanguard S&P 500 ETF (VOO 0.15%). Like its name suggests, this fund tracks the S&P 500 and includes all the stocks within the index itself.

The S&P 500 ETF is one of the most dependable investments out there. Historically, the index itself has faced countless crashes and corrections, and it has recovered from each and every one. If the market crashes again, there's a very good chance this ETF will be able to bounce back. And by buying when prices are lower, you'll reap the rewards once the market recovers and prices increase once again.

2. Vanguard Total Stock Market ETF (VTI)

The Vanguard Total Stock Market ETF (VTI 0.14%) is similar to the S&P 500 ETF, but it includes more stocks from more diverse companies.

The S&P 500 ETF includes stocks from 500 large companies, while the Total Stock Market ETF includes nearly 4,000 stocks from small, midsize, and large corporations. This provides greater diversification and can decrease your risk.

Another advantage of this fund is that it's designed to follow the market as a whole. Again, the stock market has a strong track record when it comes to recovering from downturns, so by investing in this ETF, it's likely your investments will recover as well.

3. Vanguard Growth ETF (VUG)

The Vanguard Growth ETF (VUG 0.09%) includes 285 stocks from companies that are expected to grow at a faster-than-average pace.

This fund includes the fewest holdings of the three ETFs on the list, which does make it slightly riskier. However, many of the biggest stocks in the fund are from behemoth tech corporations like Amazon, Apple, and Microsoft -- companies that are very likely to survive market volatility.

One of the primary advantages of growth ETFs is that they're designed to earn above-average returns. This particular ETF has earned an average rate of return of around 12% per year since its inception, for example. By comparison, the S&P 500 has historically earned a 10% average annual return, and the Vanguard Total Stock Market ETF has earned an average return of around 9% per year.

Investing in ETFs can be a fantastic way to build wealth with less effort, and buying during a market downturn can make investing more affordable. While nobody knows for certain whether a market crash is coming, by making a list now of the investments you want to buy, you can snag them at a discount later.