As you can tell by the description, this achievement is story related.
*** Spoiler - click to reveal ***After you gain control of your character from the crashed helicopter, your will be tasked to investigate the three other crashed helicopters. Upon completion of these objectives, will be instructed to find some ancient ruins and then ultimately finding a cave. This achievement is unlocked after shortly discovering the cave with the other survivors. You will speak with a wounded person in the infirmary and the achievement will unlock after the conversation is over.