If you think Natlan couldn't get any bigger, think again because there's an underground area hidden beneath the waterfall of Ameyalco Waters that you can uncover. Complete this puzzle to get 1 Luxurious Chest, 2 Precious Chests, and an achievement called [Retrieve the Scattered, Escaping Colors].
How To Complete Retrieve the Scattered, Escaping Colors
First of all, teleport to the waypoint near (northeast) the Artifact Domain in Natlan. Once you arrive at the waypoint, turn around and 180 degrees and you will see a small path leading somewhere. Go through this path. Walk along until you see an area between the valley and walk until you reach the location above.
Defeat the enemies there and indwell a nearby Tepetlisaurus. Use its attack to dig the buried mechanism. Give the mechanism some Phlogiston and an entrance will open.
Enter through the door and there will be multiple layer of breakable rocks. Once again, use the Tepetlisaurus' attack to break all those rocks until you reach the bottom. This will unlock the Spiritwall Cave.
Down there, you will meet your first Monetoo. Infuse it with Phlogiston so it'll start moving and you can follow it. Paimon will then mention that the graffiti is missing some parts and is dirty. What you need to do now is to find the remaining Monetoo.
Head further inside the cave until you see 3 Monetoo surrounded by Hydro Slimes. Defeat the Slimes and approach the Monetoo to calm them down and lead them to the safe place.
One of the 3 Monetoo will create a Molten Fissures and you can indwell a Tepetlisaurus to enter the Molten Fissure and find the next group of Monetoo. Up there, you will find your another one Monetoo.
For this area, make sure to keep your Tepetlisaurus underground so you can see the red glowing spot on the ground. Use Tepetlisaurus' attack on the red spot to make it explode so Monetoo can move forward.
Once you find the next Monetoo, the entrance you came in before will close and you need to interact with the Monetoo to open another entrance and meet the third Monetoo here. After that, just keep following them and choose the option "It's safe now" when it appears near them.
Now go back to the main area where you see the huge graffiti. There's only one way here so you won't get lost.
Approach the 4 Monetoo and choose the option 'Restore the graffiti' to complete the puzzle, spawn the chests, and get the achievement.