The Cu Chi tunnel network used by the Viet Cong once contained over 250 kms of tunnels.  It contained multilevel tunnels, trapdoors, weapon factories, living areas etc. It gave the Viet Cong control over an area which once extended from HCMC out to the Cambodian border. Today you can crawl through parts of the tunnel system view some of the other structures in the area, and see a lot of horrific yet ingenious traps that were using during the war.