Green Mountain is a large hill located northeast of Zelenogorsk (Ca. 1200m or 5-7 min walk in-game). On top of the hill is the Green Mountain radio tower, along with a Command center filled with Military grade loot.
A Command center is generally a point of interest due to it's valuable loot, and can therefore attract a lot of attention. So be careful when traveling across Green Mountain because the wrong people might be looting the building and there is only one way in.