Picture the Brooklyn Bridge and think of the distance from the top, past the span, all the way down to the water. Now fill that space with dirt. You will have approximately what Hamas dug through to build its deepest bunkers below the Gaza Strip. Connecting those bunkers are tunnels so numerous that their combined length is equivalent to New York’s subway. The war between Hamas and Israel was first fought on land and then from the air, but it may be decided underground. “Going into an environment where you really don’t have a good idea of what’s there is a nightmare for a military,” a Marine Corps captain who has trained in tunnel warfare told Jeff Wise. How did Hamas manage to build such a sophisticated network without Israel knowing? And can Israel possibly win an underground war?