The Bubble

"There was a lot of education going on," Zimmer said. "I realized, as a lot of people did, this was a brand new segment of the market that had a lot of potential, but I had no idea how big this would get."

Zimmer joined the business as enormous changes were taking hold in the mortgage industry. Since World War II, community banks, also called thrifts or savings and loans, had profited by taking savings deposits, paying their customers interest and then lending the money at a slightly higher rate for 30 years to people who wanted to buy homes. The system had increased homeownership from less than 45 percent of all U.S. households in 1940 to nearly 65 percent by the mid-'60s, helped by government programs such as G.I. loans.

In 1970, when demand for mortgage money threatened to outstrip supply, the government hit on a new idea for getting more money to borrowers: Buy the 30-year, fixed-rate mortgages from the thrifts, guarantee them against defaults, and pool thousands of the mortgages to be sold as a bond to investors, who would get a stream of payments from the homeowners. In turn, the thrifts would get immediate cash to lend to more home buyers.

Wall Street, which would broker the deals and collect fees, saw the pools of mortgages as a new opportunity for profit. But the business did not get big until the 1980s. That was when the mortgage finance chief at the Salomon Brothers investment bank, Lewis Ranieri -- a Brooklyn-born college dropout who started in the company's mailroom -- and his competitor, Laurence Fink of First Boston, came up with a new idea with a mouthful of a name: the collateralized mortgage obligation, or CMO. The CMO sliced a pool of mortgages into sections, called "tranches," that would be sold separately to investors. Each tranche paid a different interest rate and had a different maturity date.

Investors flocked to the new, more flexible products. By the time Zimmer joined First Boston, $126 billion in CMOs and other mortgage-backed securities were being sold annually. "Growth is really poised to take off," Zimmer thought.

After a few years at First Boston, Zimmer eventually ended up at Prudential Securities on the tip of Manhattan near the World Trade Center, selling increasingly exotic securities based not only on mortgages but also credit card payments and automobile loans.

As Wall Street's securities grew more complex and lucrative, so did the mathematics behind them. Zimmer would walk over to Prudential's huge "deal room." The room was filled with quantitative researchers -- "quants" -- a motley crew of math wonks, computer scientists, PhDs and electrical engineers, many of them immigrants from China, Russia and India. The quants built new mathematical models to price the securities, determining, for example, what borrowers would do if interest rates moved a certain way.

The industry, which came to be known as structured finance, grew steadily. Zimmer grew with it. He got married, raised two kids and climbed to the level of senior vice president, a top salesman at Prudential.

Zimmer's clients through the 1990s were mutual funds, pension funds and other big investors who dealt in big numbers: sometimes hundreds of millions of dollars. He'd get up at 4:30 a.m., be out of the house by 5, catch the 5:30 train from Princeton, N.J., be locked to his desk for 10 hours, devouring carbs -- pizza, lasagna -- and consumed by stress, but thinking nonetheless, "It was so much fun."

Chapter II

'Extraordinary' Boom

April 14, 2000. A rough day on Wall Street. The technology-laden Nasdaq stock index, which had more than doubled from January 1999 to March 2000, falls 356 points. Within a few days, it will have dropped by a third.