U.S. home values fell faster in this year's first quarter than during any quarter since 2008. In Las Vegas, that tumble has left 85 percent of homes with mortgages underwater, according to online real estate service Zillow.

Las Vegas had the country's highest rate of underwater homeowners. Nationwide, Zillow reported that 28.4 percent of homeowners with a mortgage owed more than their house was worth.

Zillow said forecasts show it is unlikely that home values will reach a bottom in 2011, prompting the company to revise its forecast. Zillow is now predicting a nationwide bottom in housing prices in 2012 at the earliest.

"Home value declines are currently equal to those we experienced during the darkest days of the housing recession. With accelerating declines during the first quarter, it is unreasonable to expect home values to return to stability by the end of 2011," Zillow Chief Economist Dr. Stan Humphries said in a statement.