In some atypical cases, bunkers were discovered buried hundreds of feet underground, with several feet of concrete protection and enough food and living space to last for weeks without resupply. Some of Hezbollah's tunnels have been thousands of meters long. Israeli sources say that North Korean instructors traveled to Lebanon in 2004 and oversaw construction of Hezbollah's underground infrastructure. In 2013, a United States federal court found that North Korea gave Hezbollah "advanced weapons, expert advice and construction assistance" in building "a massive network of underground military installations, tunnels, bunkers, depots and storage facilities in southern Lebanon."