It's no secret that U.S. home values have begun broadly declining again since last summer. Online real estate marketplace Zillow provides some information today on how severe the effect has been. Home values fell 2.6% in the third quarter -- the most since the first quarter of 2009, when the home buyer credit was put into place. Since it expired last spring, prices began to dip again. Declining home values are also putting more borrowers underwater.

How bad is it? According to Zillow:

Negative equity rose to 27 percent of all single-family homes with mortgages, from 23.2 percent in Q3. Accelerated home value declines and a temporary slowdown in foreclosures both caused negative equity rates to jump.

Think about what that means: more than one in four Americans with a mortgage are underwater. That's a pretty grim reality. Yet Zillow does mention that the slowdown in foreclosures was part of the cause, which means that some of the underwater borrowers were scheduled to lose their mortgage instead. In most cases, their foreclosure will take place this year a little late. This is hardly any consolation, but it does explain part of why the percentage of underwater homes grew more than it would have due to falling home prices alone.

Read the full press release and check out more stats at Zillow.

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