All that risk is not hidden on the balance sheets of the big Wall Street banks, as it was during the 2008 financial crisis. But it doesn’t just disappear into thin air. Like the oxygen we breathe but cannot see, it’s all around us, in hedge funds, private equity firms, and pension funds and university endowments that have been investing in risky debt, in the form of corporate bonds and other securities, to the tune of trillions of dollars.

What do those securities look like? There is, first of all, a proliferation of dodgy bank loans, issued without the covenants that are designed to protect the lender in case of default. In typical Wall Street fashion, those loans are turned into securities and sold to investors around the world. (Japanese banks are among the largest investors in these securities, called “collateralized loan obligations.”) It’s the current version of the mortgage-backed securities phenomenon that helped to sink the economy in 2008.

There is also an eye-popping amount of risk propagated by some of the biggest names in American business. Bonds rated BBB — a notch above “junk” bonds — issued by big companies such as AT&T, Verizon, G.E., G.M. and Ford, represent more than half of the investment-grade corporate bond market. That raises the ugly specter of the “triple-B cliff,” the losses bondholders will incur when the economy dips and some of those companies can’t repay their debt.

Anne Walsh of Guggenheim Partners said at an investor conference in April that the risk in the BBB market reminded her of 2008. “The overleverage is in corporate America,” she said. “This is where we feel the risk is, and right now you’re not getting paid to take that risk.”

The troubling size of this debt is not news — not even to the Fed or to the Treasury. “Business debt has clearly reached a level that should give businesses and investors reason to pause and reflect,” Mr. Powell said on May 20. “If financial and economic conditions were to deteriorate, overly indebted firms could well face severe strains.” Mr. Powell was careful to note that “the parallels to the mortgage boom that led to the global financial crisis are not fully convincing.” But without capital, layoffs and recession are inevitable, which could bring to an abrupt end a decade of prosperity.

Ten days after Mr. Powell made those comments, Steven Mnuchin, the Treasury secretary, held an executive session of the Financial Stability Oversight Council in part focused on the topic of “corporate credit and leveraged lending.”

No one knows exactly how much of the trillions of debt that’s out there is at risk of default. We do know that since 2008, the dollar amount of American corporate bonds outstanding has increased to more than $9 trillion, from around $5 trillion. Companies, universities, municipalities and governments globally have never been more indebted, with a record of $237 trillion reached at the end of 2017, 40 percent more debt than a decade ago.