For individual homeowners, being “underwater” on a mortgage – when a home is worth less than outstanding debt, or has “negative equity” – is one of the worst positions to be in, short of foreclosure.  Zillow.com, a firm that compiles U.S. real estate and mortgage information, has put together a list of the 163 largest U.S. metro areas that includes statistics on median home values, market changes and the proportion of homes with negative equity. Also included is data on short sales, which occur when real estate sells for less than the value of outstanding debt on the property.