I read that Vietnamese built miles long tunnels under the ground. Did you see something like that ?
Was pretty common. False entrances where you would find them.
Most Americans cannot read sign and so could not find entrances. (By "sign" I mean the natural patterns in the ground and plants and human marks so you can see what shouldn't be there.)
Not as easy to find entrances as all that. The holes I saw were very little, would not fit my shoulders except with arms straight over my head. Entrances about 25 cm (10") by 35 cm (14"). Maybe 40 cm wide (16") and 60 cm (20") tall inside I've been told. Not rare to be smaller. I never went into one, don't have near enough guts. They smelled terrible. Besides, I was a rear echelon guy, and have only encountered them three times. One ville with five houses had ten openings that we found. Spooky times.
Thanks for answering Iris, it's good to know we have folks who will take care of others while we're away (or even when we are here). :-)
They did build extensive tunnel systems, I never personally saw any but I did know some tunnel rats after I got home.