In 1989 Maurice Cotterell discovered the secret of the tombstone of Palenque. By making a transparent plastic copy of the tombstone and placing that on the original in different ways he uncovered many hidden images. How to place the copy on the tombstone was explained on the tombstone itself. (The mask works in the same way)
 
Here follows an overview of images that are hidden in the mask including images to elucidate.
( All off the images came from Maurice Cotterell's The Tutankhamon Prophecies and The Supergods they came on a mission to save mankind.)
This is an image of the Bat god, god of death.
The great bear assosiated with Yaotl god of the north and god of evil.
The cat can be found in the same way as the god of death.
The boy with the featered hat, an image of Pacal in his young years.
The feathered snake.
This image shows the faces of two bearded man as well as an Olmec head looking down on a baby bat.
In this picture you can see two profiles and a winged man.