After the housing crisis, subprime CDOs naturally fell out of favor. Demand shifted to a similar—and similarly risky—instrument, one that even has a similar name: the CLO, or collateralized loan obligation. A CLO walks and talks like a CDO, but in place of loans made to home buyers are loans made to businesses—specifically, troubled businesses. CLOs bundle together so-called leveraged loans, the subprime mortgages of the corporate world. These are loans made to companies that have maxed out their borrowing and can no longer sell bonds directly to investors or qualify for a traditional bank loan. There are more than $1 trillion worth of leveraged loans currently outstanding. The majority are held in CLOs.

I was part of the group that structured and sold CDOs and CLOs at Morgan Stanley in the 1990s. The two securities are remarkably alike. Like a CDO, a CLO has multiple layers, which are sold separately. The bottom layer is the riskiest, the top the safest. If just a few of the loans in a CLO default, the bottom layer will suffer a loss and the other layers will remain safe. If the defaults increase, the bottom layer will lose even more, and the pain will start to work its way up the layers. The top layer, however, remains protected: It loses money only after the lower layers have been wiped out.

Unless you work in finance, you probably haven’t heard of CLOs, but according to many estimates, the CLO market is bigger than the subprime-mortgage CDO market was in its heyday. The Bank for International Settlements, which helps central banks pursue financial stability, has estimated the overall size of the CDO market in 2007 at $640 billion; it estimated the overall size of the CLO market in 2018 at $750 billion. More than $130 billion worth of CLOs have been created since then, some even in recent months. Just as easy mortgages fueled economic growth in the 2000s, cheap corporate debt has done so in the past decade, and many companies have binged on it.