Gonna have to do what I did and hope you get lucky. Run through every spot on the fast travel station and check maps for blank spots. I found my missing hole on the second zone. If all of those zones are full, your really unlucky cause you gotta check side routes.
The last area that I had to find was an area in Three Horns that I overlooked in the far south part of the map. But if you've got all the other explorer achievements, that sucks. You ought to have it pop too.