The number of Americans who still owe mortgages worth more than their homes rose at the end of last year, according to a CoreLogic report released on Tuesday.Approximately 11.1 million homes, or 23.1% of all mortgaged homes, were underwater in the fourth quarter of 2010. That figure is up from 10.8 million, or 22.5% of homes, in the July-September quarter prior.Although the number of underwater mortgages have fallen in the previous three quarters, the report cites an uptick in foreclosures, meaning many homeowners of those underwater mortgaged houses defaulted on their payments.Underwater mortgages typically go hand in hand with falling home prices. Housing prices hit their lowest point in December in 11 out of 20 major U.S. metro areas since the housing bubble burst.The Associated Press reports that, in a healthy market, usually only around 5% of households are underwater.