Undercurrent (noun)

1. A flow of water or air beneath the surface of the sea or a river.
2. A hidden or underlying feeling or influence that is not immediately obvious.

Origin:

Late 16th century: from under- + current.

Examples:

1. There was an undercurrent of anger in his voice.
2. The undercurrent of the river was so strong that it pulled the boat downstream.
3. The undercurrent of the conversation was about politics.
4. He felt an undercurrent of anxiety in his stomach.
5. The undercurrent of the music was a deep bass.