20% of mortgages are underwater

(CNNMoney.com) -- More than 20% of the nation's mortgage borrowers owe more than their homes are worth.

At 21.5% for the third quarter, it is a small improvement over the previous quarter, when 23.3% of loans were underwater, according to real estate website Zillow.com.

This so-called negative equity is a hotly watched statistic because it is a prime predictor of foreclosure -- second only to loss of income.

"It is the paramount challenge facing housing markets," said Stan Humphries, Zillow's chief economist. "We already have had record levels of foreclosure and, combined with high unemployment, negative equity is very toxic to the market." (more)