Tales From The Eridian Slab

There are 30 altogether.  You'll spot them as you go through the game and they'll have a bright triangular icon on your map.

However you cannot decipher them until near the end of the story when you gain a specific ability to do so.   Then you'll have to go back to them all (locations guide coming soon) and interact with each one - its worth 25 Eridium apiece!