According to RealtyTrac's Year-End 2015 U.S. Home Equity and Underwater Report, 6.4 million U.S. properties were identified as "seriously underwater," meaning the combined loan amount secured on the property was at least 25% higher than the property's estimated market value. This number represented 11.5% of all properties with a mortgage.

However, seriously underwater properties at year-end 2015 were down from 6.9 million (representing 12.7% of all properties with a mortgage) at the end of the third quarter of 2015, and down from 7.1 million (also representing 12.7% of all properties with a mortgage) at the end of 2014. In addition, the number of seriously underwater properties at the end of 2015 was half of the 12.8 million (representing 28.6% of all properties with a mortgage) reported for the second quarter of 2012, when the number of seriously underwater properties peaked.

"Over the past three and a half years, the number of seriously underwater properties has been cut in half, but we continue to deal with a long tail of seriously underwater properties, and it will likely be another five years at least before most of those remaining underwater properties move into positive equity territory," Daren Blomquist, vice president of RealtyTrac, said.