an emotion, belief, or characteristic of a situation that is hidden and usually negative or dangerous but that has some effect

undercurrents of racism/anxiety/violence 潜在的种族主义／焦虑／暴力

Beneath the smooth surface of political life, there are powerful and dangerous undercurrents. 在日常政治生活平静的表面下存在着强大而危险的潜流。