Edward and Maria Moller are worried about losing their house -- not now, but in 2013. That is when the two suburban San Diego schoolteachers will see their mortgage payments jump, most likely beyond their ability to pay. Like millions of buyers during the boom, the Mollers leveraged their way into a house they could not otherwise afford by taking out a loan that required them to make only interest payments at first, putting off payments on the principal for several years. It was a 'buy now, pay later' strategy on a grand scale, meant for a market where home prices went only up, and now the bill is starting to come due. With many of these homes under water -- worth less than the loans against them -- many interest-only mortgages will soon become unaffordable, as the homeowners have to actually start paying principal. Monthly payments can jump by as much as 75 percent. The Mollers owe so much more than their house is worth, and have so few options, that they are already anticipating doom. 'I'm praying for another boom,' said Moller, 34. 'Otherwise, we'll have to walk.' Keith Gumbinger, an analyst with HSH Associates, said: 'This is going to be the source of tomorrow's troubles. The borrowers might have thought these were safe loans, but it turns out they bet the house.' After three brutal years, evidence is growing that the housing market has turned a corner. Sales in July were the highest in a year, and August gives signs of having been even better. In nearly all major cities, home prices are now rising. Celebration, however, might be premature. The plight of the Mollers and many others in a similar position is likely to weigh on any possible recovery for years to come. Experts predict a steady drumbeat of defaults over much of the next decade as these interest-only loans mature. Auctioned off at low prices, those foreclosed houses could help brake any revival in home prices. Interest-only loans are not the only type of exotic mortgage hanging over the housing market. Another big problem is homeowners with 'pay option' loans; in many of these loans, principal balances are actually increasing over time. Still, interest-only loans represent an especially large problem. An analysis for The New York Times by the real estate information company First American CoreLogic shows there are 2.8 million active interest-only home loans worth a combined total of $908 billion. The interest-only periods, which put off the principal payments for five, seven or 10 years, are now beginning to expire. In the next 12 months, $71 billion of interest-only loans will reset. The year after, another $100 billion will reset. After mid-2011, another $400 billion will reset. John Karevoll, a longtime senior analyst for MDA DataQuick, sees the plight of interest-only owners this way: 'You're heading straight for a big wall and you can't put the brakes on.' The greater the length of the interest-only period, the more years the owners have to hope for a recovery, government help, or a miracle. But a long interest-only period works against them, too. A loan that is interest-only for its first 10 years means the entire house has to be paid off in the next 20 rather than the more typical 30 years. One possible solution: start paying extra each month now to try to pay down the principal, even before the loans reset. But many overextended owners don't have the means to do that, at least not enough to make a sizeable dent in the principal. A decade ago, interest-only loans were rare. But as the boom heated up and desperate buyers sought any leverage they could, these loans became ubiquitous. They were especially popular in Florida, Nevada and above all California. In 2004, nearly half of all buyers in the state got one. The Mollers bought in 2005, paying $460,000 for their three-bedroom, thousand-square-foot house. A quick refinance a few months later supplied cash to pay debts but did them no favors in the long run. Now the house is worth perhaps $310,000. After their interest-only period is up, they expect their monthly payments to increase 20 percent if not more. 'Everyone out here always preached to me, 'Buy real estate. It's the best investment you'll ever have,' ' said Moller, who grew up in Iowa. 'Then all this stuff started crumbling and I was like, 'You're kidding me.' ' While default may be a long way off, the prospect is already dampening the couple's spending habits. They are postponing the new windows the house needs. They recently bought a 2005 Nissan Murano instead of a new car, and they have put off buying a flat-screen TV. Moller said he didn't want possessions 'and no house to put it in.' Mark Goldman, a San Diego mortgage broker, said many interest-only buyers thought they would be in control when the loans reset. 'They expected to move or refinance,' he said. 'But you can't do either when you're under water.' Among the people Goldman put into interest-only loans was himself. He refinanced five years ago to shrink his payments so he and his wife, Julie, could put their two sons through college. When the interest-only period expired a few months ago, their payments went up by 40 percent. The Goldmans have been in their house for 20 years, which means they still have some equity. Still, they are unhappy to find themselves in 'a world different than we planned for,' said Goldman, a lecturer in real estate finance at San Diego State. 'If you purchased your home with an interest-only loan between 2003 and 2006, you're cooked.' The federal government, through the finance company Fannie Mae, increased the scope of a program this summer that might help some interest-only borrowers by allowing them to refinance. But it will not help many in coastal California. Only loans owned by Fannie Mae are eligible, and during the boom Fannie had a limit of $417,000 -- not nearly enough to buy a home at the peak in a middle-class community. Dean Janis, a Southern California lawyer who bought a $950,000 home in 2004, will see his interest-only loan reset in December. He calculates that will send his payments up a minimum of 27 percent, to $3,726. A rise in rates could eventually push it as high as $6,700. 'I understand I took a risk,' Janis said. 'But I did not anticipate that the real estate market would go down 30 percent.' He talked with Wells Fargo about his options, and the lender said he had none. Homeowners with interest-only loans have a much greater likelihood of default, the First American CoreLogic figures indicate. Nationally about 18 percent of prime interest-only loans are at least 60 days delinquent. In California, the level is even higher: 21 percent, a rate exceeded only in the other bubble states of Florida and Nevada. 'The bailout is not trickling through to help many of us who have worked hard, under very difficult circumstances, to keep paying our bills,' Janis said. 'I am stuck with nowhere to go -- absent trashing my credit and defaulting.' 