The unseen is a site of mystery, fear, magic. Under the ground or the water. In the dark, or shrouded in mist. There is, too, the way the prefix “un” allows us to convey both the thing (the seen) and its absence (the not-seen), as Anne Carson writes in The economy of the unlost.
Set a timer and write for 20 minutes around the notion of the unseen, however this occurs to you.