Nearly one-third of homeowners are underwater on their mortgages, Zillow reported last week, a significantly higher number than other analysts have estimated. Zillow, a real estate information company, said nearly 16 million homeowners, or 31.4 percent, owe more than their house is worth. The company partnered with credit agency TransUnion to take into account second mortgages to calculate total loan-to-value ratios. In its most recent report, CoreLogic said 11.1 million homeowners were in a negative equity situation in March. CoreLogic relies on public data and bases its estimates on the...