The Cu Chi tunnels are one of the most depressing yet super human constructions I have ever seen. An immense network of 250 km of tunnels of 60 cm width and 80 cm height of three levels underlying much of the Vietnamese country.
Starting in the 1948 the Viet Cong guerrillas dug these tunnels by hand to protect themselves, their families and to fight against their enemies. During the Vietnam War the tunnel systems were enlarged to fight against the American invaders. The constructions were built very smartly to ensure protection, communication and supply of weapons and nutrition. The network provided everything they needed. Camouflaged entrances, traps, hospitals, schools, food and weapon caches and living quarters for numerous guerrilla fighters.
Thanks to these smart constructions and the superhuman strengths of the Viet Cong to live without daylight in the tiny, warm and sweaty tunnels, they did eventually win. But the destruction of the country and the people was massive. I took two days of my trip to visit the tunnels and several museums and I was shocked once again how brutal humans can be.
As a tourist you can also walk through a tunnel, which is actually bigger than the original tunnels. I could not make the 60 meters distance, since I was too afraid of the other tourists or myself panicking without being able to exit.