From Woomyon Camp, head south-west to the bottom left corner. It isn't highlighted on the mini map so be observant for a cave entrance.
If you fail to find it the above way. Head into Woomyon from Bichon Prov and follow the wall around (don't skip any sections of wall/cliff and again be observant for a cave opening)
There are usually insects outside the cave opening if people haven't killed them.