Could four in five mortgageholders fold? The scariest number for anyone invested in the real estate market is this: 23.2%.

That's the record-high share of mortgages that are now underwater, as estimated by Zillow.

Negative equity is the prime factor driving a record number of mortgage holders into delinquency. Delinquencies will lead to foreclosures, which will drive down home prices, creating more negative equity -- a very dangerous cycle.

In some parts of America, a gob smacking percentage of homes are underwater. In Las Vegas, for instance, four out of five mortgages are now underwater.