Yilpet, this is very old,
It looks ancient to me,
the dark yellow marks, are from the sea.
It's not in the States, It should be somewere around Middle East.
If I am still correct, this passage should be in the ancient route
from Persia to Greece, through Turkey.
What is around there?
At a mile distance, is there any small, very small, odd hill?
With height about 6 to 8 meters?
If there is one, and it's not opened, you just have hitted Jackpot!!!!
Anyway, search the place carefully, keep notes and read lots of local history books at the local library, after all it might only be just an
ancient passage.