https://www.ynetnews.com/article/b12ct00v7c
Nearly 700 tunnel shafts have been identified in Rafah, from which approximately 50 tunnels cross into Egypt. These tunnels are used by Hamas to supply itself with weapons and ammunition and could potentially be used to smuggle out of Gaza hostages, or Hamas senior operators,” Noam told the court.
Seems like there is a typo here. Other reports are of 70 tunnels, with 50 going into Egypt.
Personal theory: the reason Israel started in the north is because they knew these tunnels were there. Had they uncovered them in the beginning, there would have been much more scrutiny on Egypt's role, and Egypt and other Arab countries would have ended up being much less cooperative (than they already were).