Simple bombs or grenades can be thwarted by placing a dip in the tunnel. That's also a good way to eliminate direct fire. You can even make a tunnel dip down then up and make it crawl height so that it forces an attacker to crawl and exit at an awkward angle.

You can set up a blast door for more serious bombs.

It's a huge force multiplier for defense. Israel will never achieve their stated goal of destroying Hamas.