Between Oct. 6 and Oct. 10, the Dow Jones Industrial Average dropped nearly 2,000 points. If it kept falling at that rate, the index would hit zero in less than a month.

Of course, we won't see zero. No matter how ugly markets get, the pain we saw these past few weeks can't continue for long.

But here's the bad news: Zero may be out of the question, but that doesn't mean stocks won't plummet from here. In fact, they could fall much, much further.

And history agrees.                                                                                                    

What goes up ...
The history of long-term market downturns is pretty abysmal. When times are bad, markets don't just get drunk with fear -- they start downing vodka shots of fear.

At times like this, nobody wants to own stocks. Their palms begin to sweat every time they watch CNBC. They bury their heads in the hope that the pain will go away. They throw in the towel and sell stocks indiscriminately. In short, it gets ugly.

Just how ugly? Have a look at the average P/E ratio of the entire S&P 500 index over these three periods of market mayhem:

Period

Average S&P 500 P/E Ratio

1977-1982

8.27 times

1947-1951

7.78 times

1940-1942

9.01 times

Compare that to the average P/E ratio today of around 20 times and a seven-year average of more than 24 times, and it's pretty apparent that stocks could fall much, much further than they already have, just by returning to the lows they historically hover around during downturns.

Assuming earnings stay flat, revisiting those historically low levels could easily mean a nearly 50% decline from here. For the Dow Jones Industrial Average, that'd correlate to roughly Dow 5,000 -- give or take. Of course, I'm not predicting, warning, or forecasting -- I'm just taking a long look at history.

But what if it did happen?
What would happen to individual stocks? Here's what a few popular names would look like trading at P/E ratios of 8:

Company

One-Year Decline

Further Decline From Current Levels With P/E of 8

Yahoo! (NASDAQ:YHOO)

 

(51%)

(56%)

Apple (NASDAQ:AAPL)

 

(40%)

(60%)

Bank of America (NYSE:BAC)

 

(48%)

(62%)

Adobe (NASDAQ:ADBE)

 

(39%)

(56%)

Starbucks (NASDAQ:SBUX)

(60%)

(52%)

Pfizer (NYSE:PFE)

 

(27%)

(37%)

Schlumberger (NYSE:SLB)

 

(52%)

(33%)

Look scary? It is. And it could easily happen.

But here's the silver lining: Every one of those stocks -- heck, the overwhelming majority of stocks -- are worth much more than a measly 8 times earnings. The only thing that pushes the average stock to such embarrassing levels is an overdose of panic, rather than a good reading on what the company might actually be worth.

Be brave
As difficult as it is right now, following the "this too will pass" philosophy really does work. No matter how bad it gets, things will eventually recover. Those brave enough to dive in when no one else dares to touch stocks are the ones who end up scoring the multibagger returns.

Need proof? Think about the best times you could have bought stocks in the past: after the economy recovered from oil shocks in the '70s, after the magnificent market crash of 1987, after global financial markets seized up in 1998, and after the 9/11 attacks that shook markets to the core. As plainly obvious as it is in hindsight, the best buying opportunities come when investors are scared out of their wits and threaten to give up on markets altogether.

And that's exactly where we are.

Pick what side you'd like to be on
The next few years are likely to be quite a ride. On the other hand, the history of the market shows that gloomy, volatile periods also provide once-in-a-lifetime opportunities that can earn ridiculous returns as rationality gets back on track.

If you need a few ideas, our team at Motley Fool Inside Value is sifting through the market rubble to find those opportunities. To see what they're recommending right now, click here to try the service free for 30 days. There's no obligation to subscribe.

Fool contributor Morgan Housel doesn't own shares in any of the companies mentioned in this article. Pfizer and Bank of America are Motley Fool Income Investor recommendations. Starbucks and Pfizer are Inside Value picks. Starbucks and Apple are Stock Advisor picks. The Fool owns shares of Starbucks and Pfizer. The Motley Fool is investor writing for investors.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.