How can you map them and contents if they are underground?
Maybe the observable openings, but I expect hidden ones, fake ones, and some not open yet, waiting to be punched through.
So ... ?
It said they got them from defectors. Lots of research has gone into geophysical methods to detect the tunnels - but it is difficult. I would imagine Art Bell has done old shows on paranormal “remote viewing” of the tunnels. According to some of this guests the government actually did research into that paranormal stuff with some reasonable results.(?)