It's said that we're in the worst financial crisis since the Great Depression. Maybe. But remember the S&L crisis of the early 1980s? Or the commercial banking crisis of the late 1980s (from 1988 to 1992, 905 banks failed). Or the 1997-98 Asian financial crisis, which sent South Korea, Indonesia and other countries on a boom-bust roller coaster? All were frightening. But what distinguishes this crisis—which brought down Bear Stearns over the weekend—is that it involves the entire financial system, not just depository institutions, and it's more mystifying than any of its predecessors.

Previous financial crises so weakened the banks and S&Ls that they lost their primacy. As recently as 1980 they supplied almost half of all lending—to companies, consumers and home buyers. Now their share is less than 30 percent. The gap has been filled by "securitization": the complex bundling of mortgages, credit-card debt and other loans into bondlike instruments that are sold to all manner of investors (banks themselves, pension funds, hedge funds, insurance companies).

As a result, the nature of financial crises has changed. With a traditional "bank run," the object was to reassure the public. The central bank—the Federal Reserve in the United States—lent cash to solvent banks so that they could repay worried depositors and pre-empt a panic that would spread to more and more banks and would ultimately deprive the economy of credit. But now the fear and uncertainty center on the value of highly complex, opaque securities and the myriad financial institutions that hold them.

At the epicenter of the crisis are the now-notorious "subprime" mortgages made to weaker borrowers and subsequently "securitized." On paper the financial system seems to have ample resources to absorb losses. Commercial banks have $1.3 trillion in capital; U.S. investment banks in 2006 had an estimated $280 billion in capital—and other investors, including foreigners, may hold half or more of subprime loans. But no one knows who or how much. Recent estimates of subprime losses range from $285 billion to $400 billion or even higher. Such guesstimates, and outright ignorance, breed caution and fear.

The stunning fall of Bear Stearns reflects these realities. Bear Stearns was not a traditional commercial bank that took deposits from the public, but America's fifth-largest investment bank, which funded most of its operations with borrowed money ("leverage"). On average, the ratio of borrowed money to underlying capital for investment banks and hedge funds is about 32-1, according to a recent study. Many of these loans—commercial paper, "repurchase agreements," bank credits—are backed by the securities owned by the borrowing financial institutions.

What this means is that if lenders became worried about the worth of those securities, they might ask for more collateral or pull their loans. In effect, that's what happened to Bear Stearns. Deprived of its credit lifeblood, Bear Stearns either had to collapse or be purchased by someone with credit and clout. That someone was JPMorgan Chase, which, with substantial help from the Fed, bought Bear for almost nothing: $236 million for a firm valued at $20 billion in 2007.