Tunnels have been used in warfare for centuries. The first known use of the tactic dates back to the Ancient Romans and Persians, who would tunnel under barricades and walls to enter city walls and fortresses.
World War I saw the use of tunnels loaded with so many explosives that they changed the landscape of Germany. North Korea and Vietnam used increasingly extensive and complex tunnel systems during times of war to evade and outsmart their enemies.
More recently, Hamas militants have built a series of underground tunnels known as the "metro." The tunnels are reinforced with concrete and iron bars, and some are big enough to fit a truck through.
"Having tunnels as part of a landscape of warfare makes it very, very difficult for an invading army to be successful," Howard Stoffer, a professor of national security at the University of New Haven, told Insider. "Because they provide tremendous advantages to the defense, and I think they're [Hamas] using them very effectively."
Tunnel warfare provides safety from air attacks and wards off detection. With an expansive and reinforced tunnel system, Israel faces a major challenge in rooting out Hamas.