Tunnels

One of the things that will make an Israeli ground attack on Gaza so perilous will be the tunnels Hamas has dug under the city. Israel can bomb them but they are built under civilian structures, including homes, hospitals, churches, etc. And if the Israelis want to find the hostages, they will have to search those tunnels in person.

Wikipedia has a great article on the tunnels under Gaza, with some videos, animations, and photos of them ... Palestinian tunnel warfare in the Gaza Strip

This reminds me of the Vietnam war ...

The tunnels were used by Viet Cong soldiers as hiding spots during combat, as well as serving as communication and supply routes, hospitals, food and weapon caches and living quarters for numerous North Vietnamese fighters. The tunnel systems were of great importance to the Viet Cong in their resistance to American forces, and helped to counter the growing American military presence ... - Củ Chi tunnels. And also Vịnh Mốc tunnels

More: What to know about Hamas' tunnel system beneath Gaza