Sure, the rally over the past few months has been a fun ride, but how quickly we forget: Between Feb. 9 and March 9, the Dow Jones Industrial Average dropped more than 1,700 points. Repeat another of those plunges, and the "Dow's going to zero" camp might start gaining attention again.

Of course, we're not going to zero. No matter how ugly the markets get, the ferocity of what we've been through over the past 18 months can't continue for long.

But here's the bad news: Just because zero is out of the question 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 hideous. When times are bad, markets don't just get drunk with fear -- they start downing vodka shots of fear. When panic sets in, nobody wants to own stocks at any price. Investors' 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, things get really, really ugly.

Just how ugly? Have a look at the average price-to-earnings 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

1947-1951

7.78

1940-1942

9.01

And while stocks have plummeted over the past year, so have corporate earnings. With Standard & Poor's predicting the S&P 500 will earn $28.65 per share in 2009, the index currently trades at more than 30 times earnings. Compare that with the above table 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 50% decline from here. For the Dow Jones Industrial Average, that could easily mean Dow 5,000, or worse. Now, 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 Return

Decline From Current Levels
With P/E of 8

Akamai (Nasdaq: AKAM)

(41%)

(68%)

Yamana Gold (NYSE: AUY)

(43%)

(43%)

JPMorgan Chase (NYSE: JPM)

(9%)

(72%)

Duke Energy (NYSE: DUK)

(16%)

(46%)

Google (Nasdaq: GOOG)

(24%)

(73%)

US Bancorp (NYSE: USB)

(38%)

(42%)

eBay (Nasdaq: EBAY)

(39%)

(37%)

Look scary? It is. And it could easily happen. It has in the past, and it'll probably happen again.   

But here's the silver lining: Every one of those stocks -- heck, the overwhelming majority of stocks -- are worth much more than a pitiful 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 just about where we've been over the past few months.  

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 stock 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.

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This article was originally published Oct. 18, 2008. It has been updated.

Fool contributor Morgan Housel doesn't own shares in any of the companies mentioned in this article. Akamai and Google are Motley Fool Rule Breakers picks. eBay is a Stock Advisor and Inside Value selection. Duke Energy is an Income Investor choice. The Fool has a disclosure policy.