That is amazing. The Cu Chi tunnels in Vietnam (about 20km north of Saigon/Ho Chi Minh City) are in area that is limestone like in the video. It is stable and is almost like concrete when they were finished. They went down 3-4 levels and included hospitals, barracks, kitchens, armory, food storage etc. I’m 6’1” and could only go a few yards through the tunnel as it steadily narrowed and twisted. It is amazing what can be built by hand with determination, skill and hard work. The mound by my foot is a fresh air vent disquised as a termite mound.
