What does underground love mean?

This is a secret relationship. A relationship that only two people in love know and no one else knows. Talking about a secret, unknown relationship is going on in secret Although falling in love is a matter between two people, if you don't want a third person to know about it, or to avoid affecting your normal work, you don't want others to know about it, it belongs to an underground romance. Only two people in love know about underground love, and others don't. This kind of love may be true love or ambiguous. Students fall in love behind their teachers' backs, and colleagues fall in love behind their bosses. The rest is hard to describe. )