Serious strains are starting to appear in the $1.2 trillion market for loans to high-risk companies, which have borrowed record sums in recent years as investors chased bigger yields.

The market, which survived the 2008 financial crisis, has become overstretched since then, say regulators and economists, who worry that it is now so big and risky its problems could amplify any economic damage caused by the coronavirus crisis.

“What I’ve always worried about is that the existence of overleveraged corporations will exacerbate a downturn that occurs for any reason,” said former Fed Chairwoman Janet Yellen in an interview.

Years of low interest rates and easy credit have allowed companies across the board to borrow big, building a record $10 trillion mountain of debt. Lenders expect the vast majority of that money to be repaid on time.

The epicenter of risk involves a subset of that total: $1.2 trillion in leveraged loans, junk-rated debt secured by corporate assets much like mortgages are backed by homes. The market has exploded, ballooning by almost 50%—or $400 billion—since the start of 2015, as investors desperate for the high interest payments these loans provided threw cash at borrowers.