Any person with a moderate education knows there are devices that can spot air pockets underground. Obviously Israelis will pass these over the length of the border every few weeks and identify each tunnel before it is completed. It's sort of the Darwinism of terrorists.
Israeli teen lightly wounded from stone throwing in West Bank settlement of Kiryat Arba (Haaretz)
from the article: Leak leads to discovery of mega-tunnel from Gaza to Israel