I have never wanted to crack an area of a game as badly as this one:
This one is not easy. You will find yourself enduring a bit of positive stress, excitement and frustration, induced by the time component of the puzzle.
To unlock the casket, you have to solve more than a dozen puzzles, while racing trough the caves. You will have less than 30 seconds per puzzle. A single, small mistake might make you have to start over. When the second songs starts thumping trough the speakers - faster and faster, louder and louder, you know you're in trouble, for you are running out of time.
Spoiler Free Hint
All puzzles are connected, but some are more connected than others. Understanding this hint will save you a bit of time.
Oh, the joy
Beating this puzzle has been the most rewarding experience of all the puzzle games I've played so far.
About "The Witness"
You're exploring a beautiful island, filled with puzzles and deeply satisfying, hidden treasures. The game is exceptionally well designed, and it will teach you how to think as you progress.
If you're going to play it, take your time to explore, observe carefully, and have fun.