Last week, First American CoreLogic released a report of nationwide housing statistics. It focuses on “negative equity,” or homeowners who owe more on their mortgage than their house is worth. We refer to these folks as “underwater.”Nationwide, 11.2 million, or 24% of all mortgages, are now underwater. That’s horrible — but it isn’t the scariest finding.

The report also details those who are really, reallyunderwater. For example, 10.4% of all borrowers are underwater by 25% or more. That’s a total of 4.9 million homeowners, whose aggregate negative equity totaled $656 billion, or almost 5% of GDP.

Now factor in last summer’s findings from a trio of economists who calculated homeowners’ propensity to walk away — to just stop paying their mortgages — as the level of negative equity rises. They found that when a homeowner is $50,000 underwater, 9.38% declare intentions to walk away. At $100,000, 25.81% call it quits. At $200,000, the number grows to 41.23%. (Since the social acceptance of walking away has surely grown since last summer, it’s reasonable to think these numbers are now higher.)

Simple math from here: 4.9 million homeowners with $656 billion of negative equity gives us an average of $134,000. Thus, we should roughly expect something like 33% of these homeowners, representing $215 billion of that negative equity, to eventually walk away. That’s one-fifth of a trillion dollars of home loans that could vaporize before long.

Who’ll bear this burden? The three big private mortgage lenders — Bank of America (NYSE:BAC), Wells Fargo (NYSE: WFC), and JPMorgan Chase (NYSE: JPM) — are somewhat vulnerable, thanks to their acquisitions of nimrod lenders: Countrywide, Wachovia, and Washington Mutual, respectively.

But remember that the largest bearers of home mortgage risk are Fannie Mae (NYSE:FNM), Freddie Mac (NYSE: FRE), and the Federal Housing Administration (FHA). These three own or guarantee far more than half of all the mortgages in this country, and they’re backed by you, me, and every other taxpayer you know.

Last December, the administration agreed to cover an unlimited amount of losses at Fannie and Freddie for the next three years. At this rate, they’ll need to.