Workers trying to save for retirement already face huge obstacles. With corporate downsizing, the shaky stock market, and rising costs of living, managing both to set money aside and invest it successfully is harder than ever.

In the past, workers could expect some help from employers in the form of a pension plan. Yet as traditional pensions have started going by the wayside for younger workers, those closer to retirement face an uncertain future because of one thing: whether their employers will be able to meet the pension obligations they've promised. For many companies, workers won't like the answer they get.

The pension gap: worse than 2008
A recent report from Credit Suisse took a look at the status of corporate pension plans in the United States. The news was astounding: The amount that companies have set aside in plans is about $388 billion less than the liabilities they owe to current and future pensioners. As a result, pension plans overall are only about 77% funded.

But two other facts are even worse. First, as recently as 2007, these pension plans were fully funded, showing just how quickly things deteriorated for corporate America. Even more alarmingly, the current funding status is even worse than at the end of 2008, when the nation was in the middle of the financial crisis and market meltdown. At that time, the pension gap was only $326 billion, or 79% funded.

So how did pensions get so underfunded even as the stock market rests comfortably higher from its 2008 ending levels? Much of the answer lies in the same culprits that have plagued retired investors: low interest rates. According to Credit Suisse, total pension liabilities rise by about $45 billion for every quarter-point drop in the interest rate used to discount future payments. With Treasuries at unprecedentedly low yields, accounting rules force pension plans to save more to compensate for lower projected returns.

The hardest-hit companies
For several companies, the problem is far worse. Ford (NYSE: F) and Lockheed Martin (NYSE: LMT) face pension shortfalls of about $17.8 billion and $14.3 billion, respectively. Not only are those amounts substantial, they also represent a sizable portion of their overall market capitalizations.

Several smaller companies face even tougher pressure. AK Steel (NYSE: AKS), Goodyear Tire & Rubber (NYSE: GT), and US Steel (NYSE: X) all have pension shortfalls that equate to more than half of their market caps, with Goodyear at more than 75% and AK Steel facing a staggering 92% shortfall. Nor is the problem limited only to industrial stocks; grocery retailer SUPERVALU (NYSE: SVU) and business printing services provider R.R. Donnelley (Nasdaq: RRD) also appear on the list of companies whose pension gaps amount to at least a quarter of the total value of their outstanding shares.

What it means for you
Obviously, those who expect pensions from companies facing shortfalls have to be concerned. Although the Pension Benefit Guaranty Corporation insures a certain base level of benefits even if a company's pension plan runs out of money, the solvency of the PBGC itself has raised concerns in recent years. There's always a chance retirees will have to accept lower benefits. To protect yourself, make sure you have at least some savings of your own outside of your pension.

In the meantime, shareholders of companies that have pension shortfalls could see big charges to earnings as those companies scurry to cover mounting liabilities. Although some analysts will dismiss those charges as one-time events, they still represent huge amounts of money -- money that those companies won't use for more shareholder-friendly actions like dividends or share buybacks.

Investors who ignore the impact of pension plan shortfalls on their stocks could get a nasty shock in the coming years. Unless interest rates and stock returns rebound, pensioners need to make sure they're protected from the potential consequences.

Pension plan liabilities are just one hidden trap to watch out for with stocks. Learn more by watching this video message about the biggest threat to your wealth and how you can protect it.