More than $1 trillion of outflows in 12 months would roil just about any segment of the global financial markets. For prime money market funds, a corner of the U.S. money market industry that had only $1.4 trillion of assets to begin with, it looked like a fatal blow.

Yet almost two years after reaching their low point, prime funds, which buy certificates of deposit and company IOUs, are staging a surprising resurgence. They've attracted inflows for 10 consecutive weeks through Sept. 12, the longest stretch since early 2009, according to Investment Company Institute (ICI) data. At $534 billion, the industry remains a fraction of what it was before sweeping changes to U.S. money markets prompted a mass exodus starting in late 2015. Yet the steady stream of cash suggests a path forward.