Shipwreck Beach - Heading for the Endurance Wreck

After exiting the challenge tomb, turn left and cross the wooden bridge.

If you didn't kill the guy with the shotgun here earlier, he'll appear at the end of the bridge. Take care of him, loot his body, and continue forward (southeast). Mind the drop-off ahead, though.

When you come to the cliff's edge, jump onto the wooden beam, walk across it and then hop forward onto the platform. This is the crow's nest of the old shipwreck on the beach below, near where you found the 10th GPS cache.

Stand near the anchor post and fire a rope arrow into the climbing wall on the cliff ahead.

Slide down the line and press Interact to latch onto the wall with your axe. Then climb onto the ledge above.

Inside the little shelter is another document (3/4), one of the Ancient Scrolls set, and on the roof is a salvage crate. You can't open the reinforced gate on the right yet because you don't have the right equipment. You'll be able to get in there on the way back from the Endurance. For now, go through the tunnel to the southwest to reach the next area.