Glancing at the One Deep Breath prompt "the unseen," the first reference that came to mind was Antoine de Saint Exupery's classic children's book Le Petit Prince. In it, the hero - "the little prince" - describes his love for a particular rose: "It is only with the heart that one can see rightly. What is essential is invisible to the eye."
Inspired by the little prince, a haiku written on the Paris metro:
intangible and unseen,
but its power strong