Geothermal Valley - High Diving
Make 4 high dives.
High Dive #1
There's a path by the Ruins Encampment that will take you behind the nearby waterfall. The first high dive plank is at its end.
High Dive #2
You can do this dive when tasked with lighting the signal fire. See the diving board across the gap? As you climb, you'll have a chance to drop to it.
High Dive #3
There's a wooden watchtower along the icy water at the southern end of the valley. You'll need to complete the side mission Defensive Strategy before you can climb it, but the third diving board will be at the top when you do.
High Dive #4
You can't complete the final dive until returning from the Flooded Archives - this opens the ridgeline in the northwest corner of the map. Check at the top of the waterfall to find it.