A current below the surface of water, sometimes flowing in a contrary direction to that on the surface
{i} stream that flows beneath the surface; concealed tendency or intention often opposed to the one outwardly displayed
If there is an undercurrent of a feeling, you are hardly aware of the feeling, but it influences the way you think or behave. a deep undercurrent of racism in British society
Hence, figuratively, a tendency of feeling, opinion, or the like, in a direction contrary to what is publicly shown; an unseen influence or tendency; as, a strong undercurrent of sentiment in favor of a prisoner
An undercurrent is a strong current of water that is moving below the surface current and in a different direction to it. Colin tried to swim after him but the strong undercurrent swept them apart