In order to unlock this achievement, you must find the hidden tunnel on the Caspian Border map. I've heard quite a few people say the achievement won't unlock for them. What I have to say to that, when you do find the tunnel, enter from the side that's under water, and run the entire length. I've also heard you need to be the first one in the tunnel for it to count, so if someone has been in there prior to you, it won't unlock.
I've attached the YouTube video I used below to show you exactly where the tunnel is.
Video credit goes to ShockblastMedia.I hope this solution helped! Leave a comment letting me know if there is any room for improvement! Thanks.