One of the surprises Israel got when they went after Hezbollah in 2006 was the extensive underground tunnel systems that had been dug under some of the hills in southern Lebanon. These tunnel complexes were a major reason for Hezbollah being able to fire 4,000 rockets into northern Israel, despite Israeli air power, artillery and, finally, ground troops. So Israel has trained some of its troops to fight underground, borrowing some tricks developed by American "tunnel rats" during the Vietnam War.
Israel obtained details of the Hezbollah underground complexes, many consisting of several kilometers of tunnels, connecting storage areas, numerous exits (often into bunkers or homes, schools and other public buildings) and rocket launching positions. While Israel has bought thousands of penetrator bombs, many of the Hezbollah tunnels and storage areas are too far (over 20 meters) down, or their location is not precisely known.
So Israel has built a replica of some of the Hezbollah tunnels, and is training special assault troops to operate down there, and use explosives to cripple the tunnel systems. The Israelis know where a lot of the exits are, and plan to seize these locations, send the tunnel assault teams down, and then continue on to another exit. The tunnel assault teams can also use sensors and robots to survey the extent of the underground system a particular exit is connected to. The tunnel assault troops also have thermal sights and sensors for detecting Hezbollah gunmen and booby traps. But the Israelis know that there will still be surprises. So the tunnel assault troops are trained to deal quickly with what is known, and be ready for any unknowns.