Hidden Treasure is the 2nd layer of the mine in Mining Simulator 2 and the first actual layer of The Overworld. This layer of the mine is the easiest to get to & begins when you reach a depth of 97. The layer continues until depth 396.
In the Hidden Treasure layer lobby, you access a free chest that grants a certain number of coins based on your multiplier. The chest can be claimed every 15 minutes or respawned instantly for 25 Robux.