The Cu Chi Tunnels were devised by the Viet Cong and were the base of their operations during the Tet Offensive. There were up to three levels of tunnels in some sections. Along with there were were larger rooms built into the earth that were for cooking, supplies and sleeping. There were even air tunnels that led to huge termite mounds. Holes were drilled into the mounds in inconspicuous places so that the enemy would never know the true purpose of the termite colony.
In the tour that we took you could actually have a go at getting into the tunnels from the original small openings. This guy on our tour took up the offer. The thought of being in such a small narrow space creeps me out so I did not partake.
There were other small entrances like this one that I did take to go down. The tunnels were narrow and I had to almost crawl to get out. We did this for 20 meters which was plenty for me. Thankfully there were also lights along the way as our tour guide went much faster than I would have liked. I booked it as to keep up.