Hezbollah’s subterranean network highlights the enduring features that characterize underground warfare. Three in particular stand out. First, tunnels are often a major equalizing factor for an outmatched defending force. During the campaign to recapture vast territory seized by ISIS in Iraq and Syria, almost every major battle took place in cities, where ISIS forces had used existing and newly created subterranean spaces to turn urban areas into fortresses, undergirded by networks of tunnels. Second, in many cases there is virtually no alternative to sending people into tunnels, despite wishful thinking to the contrary (US Army doctrine on subterranean operations warns that “entering and fighting in a subterranean environment is extremely high-ris and units should avoid these fights whenever possible”). In April 2017, the US military dropped the largest nonnuclear bomb in its arsenal on an ISIS tunnel complex in Afghanistan that was too deep to be destroyed by less powerful bombs. Not every military force has such weapons at its disposal, and only on battlefields entirely devoid of noncombatants could its use be contemplated. In other cases, securing tunnels means entering them. And third, for military forces, there is no substitute for experience. Underground environments are so deeply unique that lessons must be learned practically. Israeli forces have benefited from more extensive experience than arguably any other military in the world. For others—like the US military—deliberate efforts to learn from the experience of others and retain lessons from its own history are vital.

https://mwi.westpoint.edu/israels-campaign-against-hezbollah-and-the-fight-for-southern-lebanons-tunnels/?s=08

(http://www.autoadmit.com/thread.php?thread_id=5607114&forum_id=2:#48167797)