Fade to Silence

Once you have cleansed the Peaks Outpost, (to the North East from the refuge), you will notice a pathway leading further North East. This is a railway tunnel and the Prairie is the area on the other side of that tunnel.

The achievement popped as I was leaving the tunnel.

For those playing co-op, the achievement is host only.

For anyone who plays this on coop, the person that joins the host will not get the achievement for discovering the Prairie when found.