After making significant progress through the main story and completing the Flooded Archives
chapter, your goal will be to proceed to the observatory at the top of hill near the waterfall in the Geothermal Valley
Just a bit down hill from the base camp at the top of the hill, off to the right hand side (if ascending), you'll see a small group of people sitting next to a tombstone. Walk up to them for the achievement to unlock.
You can replay Flooded Archives
in Expeditions if you missed this opportunity.