The mountain bike trails in that area were started about 10 yrs ago by the Coos Cycling Club on "Power Island", and their abundant ambition added the Undermine Loop a couple years later. This was one of the most popular areas when they first started developing their trail network, but eventually fell out of favor with many riders as new milage was added on the Gorham Land Company lands to the north of Pine Mountain.
I'm commenting here to provide a link to probably the best map of the immediate area that is available
https://static1.squarespace.com/stat...228_4print.pdf
And if you are curious their website
https://cooscyclingclub.org/