(Exodus, Ibid.) "The desert has closed upon them": When Israel saw the sea raging, they turned their faces to the desert (to escape), and the Holy One Blessed be He confronted them with wild beasts, which did not let them pass, viz.: "He has closed 'sagar' the desert upon them," "sagar" alluding to wild beasts, as in (Daniel 6:23) "My G–d sent His messenger and closed the lions' mouth and they did not wound me."
The Desert Has Closed Upon Them When Israel Saw
Mekhilta Tractate Vayehi Beshalach 2:7