Mysterious Maps
A 
Item 25736 starts a treasure hunt. The map never tells you where the treasure is or how far away it is, only which way to go from where you stand, so you find it by reading the map from different places until the directions close in on one spot.
Starting a hunt
- Click "Read" on the Item 25736.
- A window asks you to confirm "Start treasure hunt". Pick Yes.
- The chat says "You have begun a treasure hunt" and the map becomes a
Item 25737.
There is also a 
Item 27701, with its own 
Item 27702.
Reading the map
Each read gives one direction from where you are standing:
The treasure should be South-west from here!
There are eight directions: North, North-east, East, South-east, South, South-west, West and North-west. There is never a distance.
One read only tells you which way to head. Reads from two places far apart, with directions that cross, narrow it down a lot more. Close to the treasure the direction can swing from East to West as you step past it, which tells you you are almost on top of it.
Digging
Bring a Spade. Dig on the spot and the chat says:
Caskets have appeared! Only one is real and gives a bonus reward.
Three caskets appear around you. Only one of them is the real one.
Rewards
The Currencies page lists Verdant Fragments as a Treasure Map reward. What the real casket gives has not been recorded here yet. Contribute if you have it.
RXLite
RXLite has a Treasure Maps helper. It keeps each read with where you stood, names the place to dig once the reads close in, suggests where to read next, and marks the spot on the ground. Spots other RXLite players dug up before are shared, so a hunt can start from them. It only reads the map's own messages and never moves or clicks for you.