The phrase "strong undercurrent" means a powerful force or influence that is hidden or not easily noticeable, but still affects a situation or a group of people. Full definition
An equally strong undercurrent of anxiety percolates around English forays into non-lawyer ownership of law firms. (slaw.ca)
The waves can be a bit rougher here for swimming, with strong undercurrents (areas are usually marked by red, no - swim flags). (bali-indonesia.com)
Thus, investors should be cognizant that focusing only on aggregate data would likely miss strong undercurrents that would later prove destabilizing. (kekselias.com)