In the Voracious Canopy zone on Eden-6, leave the lower science dome section of the map, and look left for a drop into a narrow cave. It leads to the Eridium Marker collectible, but ignore that. This cave contains lots of Eridium Crystals. Simply smash and collect them, then exit to the menu and reload to the New-U Station to spawn the Eridium Crystals again. Repeat this as many times as desired. Note: You will get even more Eridium by doing this in Mayhem mode.