The Ilagan Japanese Tunnel is a war tunnel that was part of a military base built by the Japanese government as headquarters for its soldiers during World War II. It is found in barangay Santo Tomas in Ilagan, Isabela. It is one of the few remaining tunnels in the province. It measures about 40 meters in length and slightly over three meters in width and height. The real size of the tunnel, however, has yet to be determined as the site has yet to be fully checked and explored.