GAZA—A quarter of a mile from a civilian border crossing between Israel and northern Gaza lies what Israel’s military says is the largest tunnel discovered in the enclave. It is large enough that large vehicles can drive through it, and yet, until recently, Israel didn’t know the tunnel reached right up to its border.

Israeli troops uncovered the tunnel exit buried under a sand dune a few weeks ago. Israeli officials believe that the tunnel, up to 50 meters deep at points, and 2½ miles long, took years and millions of dollars to build and was meant to facilitate a large-scale attack on Israel.