1. See PG. But Iv'e heard that it's extremely more difficult to build that deep, as you need oxygen etc.
2. OK.
3. You can't fight terror with terror.
4. Brilliant! (If it indeed can be implemented, and if Israel does not know where they are anyway.)
1. This is supposedly already in the works, it would be very hard for them to dig under the Israeli tunnel, they would need special equipment, plus I am sure the IDF would be able to sense downwards 10-20 ft since there is very little movement underground.
3. This is not fighting with terror, this is just learning some tricks from them, it seems like the ground in that area is very easy to dig into, let the IDF make their own tunnels.
I am curios if Hamas discovered any ruins or archaeological finds during their digs. Asara Kadisha should protest against Hamas whenever they dig in case a body may be discovered.