The 3,000-year history of tunnel warfare shows how hard it's going to be for Israel to root out Hamas

• Tunnel warfare has been used since the 9th century BC.
• Tunnels provide advantages in warfare by providing cover and secrecy. 
• Most recently, Hamas militants have been using tunnels in the conflict with Israel. 

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.