Sforno on Exodus 8:17/ספורנו על שמות ח׳:י״ז

1וגם האדמה אשר הם עליה, a reference to the soil on which the houses stood which would become full of snakes and other creepers of the kind that inhabit the lower levels of the earth beneath its surface. The plague would result in the Egyptians feeling insecure in their homes even when they had shut every exit tightly.