About one-third of all homes with mortgages in metro Atlanta stayed upside down in the third quarter, CoreLogic reported Tuesday.In metro Atlanta, 34.5 percent, or 420,160, of all residential properties with a mortgage were in negative equity in the third quarter, according to CoreLogic. This compares with 34.7 percent, or 423,130 properties, in the second quarter. Negative equity, often referred to as “underwater” or “upside down,” means borrowers owe more on their mortgages than their homes are worth. Negative equity can occur because of a decline in value, an increase in mortgage debt or a combination of both.In Georgia, 30 percent, or 488,310, of residential properties with mortgages were underwater in the third quarter, compared with 30.2 percent in the second quarter. This was the fifth-highest state negative equity rate in the third quarter.Nationally, 22.1 percent, or 10.7 million, of all residential properties with a mortgage were underwater at the end of the third quarter. This was down a bit from 22.5 percent, or 10.9 million properties, in the second quarter.