Having negative equity, which is also known as being “underwater” on a mortgage, makes homeowners more likely to end up in foreclosure. It restricts a borrower’s ability to refinance or buy another home, which in turn stifles demand for housing. It even reduces the flexibility of the labor market, since underwater homeowners are less willing to leave town to take a different job, says Stan Humphries, the chief economist at Zillow.

“We have never had negative equity like this at the national level in as many different regions as we have now,” Humphries says. To get a better sense of the cities with the greatest concentrations of negative equity, Zillow provided U.S. News with data that detail the percentage of mortgage borrowers who are underwater in 142 distinct markets throughout the country.

Based on this research, we compiled the following list of America’s most underwater housing markets. (Please note: We chose no more than one city per state.)

Las Vegas

Las Vegas was ground zero for the housing market’s historic boom and bust. Loose lending standards and speculative fervor helped send home prices surging more than 104 percent from 2002 to their 2006 peaks, according to Moody’s Economy.com.

“We all knew in our hearts it was unsustainable and there had to be a correction,” says Larry Murphy, the president of SalesTraq. That correction came as the housing bubble popped and the economy tanked: Home prices in Las Vegas fell more than 56 percent from 2006 to the third quarter of 2009. This steep decline has pulled a vast swath of mortgage borrowers underwater.

“If you bought a home in Las Vegas since 2004 up to about 2007, whatever you bought–I don’t care if you bought a big house or a little house, in a great neighborhood or a crummy neighborhood–it’s worth about half what you paid for it,” Murphy says.

More than 81 percent of single-family home mortgages in Las Vegas had negative equity in the fourth quarter of 2009, according to Zillow. And it may take 20 years for some of these home values to climb back to the levels they hit at the peak of the housing boom, Murphy says.

Merced, Calf

The housing crisis that has rocked Merced, Calf., was initially linked to rising property values in relatively nearby metropolitan areas like San Francisco. As real estate became increasingly unaffordable in the bigger cities, many would-be home buyers started exploring options in smaller markets, such as Merced.

“A number of people said, ‘Hey, I have got a couple of choices: I can get a 1,000-foot condo in San Francisco, or I can move east and I can get myself a fairly significant home for the same price,’ ” says John Walsh, the president of DataQuick. Although this trend increased real estate demand in Merced, prices appreciated even faster as exotic mortgage products and investor interest hit the market.

Area home prices jumped nearly 129 percent from 2002 to 2006. But after the euphoria subsided, home prices crashed more than 72 percent through the third quarter of 2009. This rapid deflation dragged about 64 percent of single-family home mortgages underwater by the fourth quarter of 2009, according to Zillow. Walsh says it could be 10 to 20 years before Merced home prices reach former peak levels.

Phoenix

As exotic mortgage loans and investor demand swept through the market, home prices in Phoenix jumped more than 101 percent from 2002 to their 2006 peaks. Jay Butler, an associate professor of real estate at Arizona State University, says many people who purchased property in Phoenix during the boom felt pressure to get in on the action. “You had [real estate] seminars all over the place, you had ‘flip this’ shows,” Butler says.

“You were constantly being fed a barrage that if you weren’t actively participating in this thing, you were not only denying yourself a great bit of wealth but your kids [and] your grandkids.” But once the music stopped, the housing market in Phoenix was clobbered.

Home prices dropped more than 52 percent from their peaks through the third quarter of 2009. And as of the fourth quarter of last year, nearly 62 percent of single-family home mortgages were underwater, according to Zillow.

Credit ratings agency Fitch Ratings projects that about two-thirds of borrowers with permanent modifications under the Obama plan will default again within a year after getting their loans modified.

Obama administration officials contend that borrowers are still getting help — even if they fail to qualify. The administration published statistics showing that nearly half of borrowers who fell out of the program as of April received an alternative loan modification from their lender. About 7 percent fell into foreclosure.

Another option is a short sale — one in which banks agree to let borrowers sell their homes for less than they owe on their mortgage.

A short sale results in a less severe hit to a borrower’s credit score, and is better for communities because homes are less likely to be vandalized or fall into disrepair. To encourage more of those sales, the Obama administration is giving $3,000 for moving expenses to homeowners who complete such a sale or agree to turn over the deed of the property to the lender.

Administration officials said their work on several fronts has helped stabilize the housing market. Besides the foreclosure-prevention plan, they cited government efforts to provide money for home loans, push down mortgage rates and provide a federal tax credit for buyers.

“There’s no question that today’s housing market is in significantly better shape than anyone predicted 18 months ago,” said Shaun Donovan, President Barack Obama’s housing secretary.

The mortgage modification plan was announced with great fanfare a month after Obama took office.

It is designed to lower borrowers’ monthly payments — reducing their mortgage rates to as low as 2 percent for five years and extending loan terms to as long as 40 years. Borrowers who complete the program are saving a median of $514 a month. Mortgage companies get taxpayer incentives to reduce borrowers’ monthly payments.

Consumer advocates had high hopes for Obama’s program when it began. But they have since grown disenchanted.

“The foreclosure-prevention program has had minimal impact,” said John Taylor, chief executive of the National Community Reinvestment Coalition, a consumer group. “It’s sad that they didn’t put the same amount of resources into helping families avoid foreclosure as they did helping banks.”

Long Term Debt – Financing used to purchase or improve assets such as plant, facilities, large equipment and real estate.

Maturity – A loan’s maturity is the life of the loan; that is, how long you have to repay the loan. It usually applies to term loans and not lines of credit.

Multi-Lender Environment – Numerous lending institution sharing the same site and information to provide instant financing to small businesses.

Personal Guarantee -A guarantee that the primary owner will assume personal responsibility for repayment of the loan, should the company not repay the loan.

Prime Rate – The rate a lender charges its best customers. The rate is calculated differently by each lender.

Revolving Credit – It is the same thing as a line of credit: an amount of money, which a business can borrow against at times it needs capital. Often accessed by check, ATM, or business card.

SBA Loan – Loans to small businesses unable to secure financing on reasonable terms through normal lending channels. The program operates through private-sector lenders that provide loans, which are guaranteed by the Small Business Administration (SBA) — the SBA has no funds for direct lending or grants.

Secured Loan – A loan secured by specific collateral. Creditor may foreclose and seize the specific property that is collateral to satisfy an unpaid secure loan.

Small Business Administration -Established by Congress, the SBA provides financial, technical and management assistance to help Americans start, run, and grow their businesses.

Short Term Debt – Financing used to secure cash for accounts payable and inventory.

Subsequent Draw Fee – It’s a fee that the financial institution may charge each time you use the line of credit after the initial use.

Term Loan – A loan for a specific amount of money. It has either have a fixed or variable interest rate, matures in between one and ten years and has a set repayment schedule.

TransUnion Corporation – One of three leading providers of personal credit information.

Unsecured Loan – A loan granted upon the good credit of the borrower. No collateral involved.

Variable Interest Rate – An interest rate that changes during the life of a loan.