Hezbollah built a sophisticated network of tunnels with North Korean assistance, with close resemblance to North Korea's own network of tunnels in the demilitarized zone separating the two Koreas. The underground network included twenty-five kilometer tunnels, bunkers, fiber optics communication systems, and storerooms to hold missiles and ammunition. Its capabilities were extended by Iranian supply of advanced weaponry and in-depth training of Hezbollah fighters. Between December 2018 and January 2019, the Israeli military destroyed six tunnels built by Hezbollah along Israel's border during Operation Northern Shield.