Reread the sources and consider the expression of ‘underground’ bases to reference a fifth column or parallel power structure being built from within along the lines of freemasonry or the crafts and spiritism.
If you are searching for underground caverns with battalions of fighting men, you will be looking for the wrong objects.
" He showed me one of the cars in the rail yards in North Portland. He was right. If you multiply 107,200 times 143 times 11, you come up with about 15, 000, 000. This is probably the number of people who disagree with the Federal Government."
LOL - I think they need a few more cars.