Facing larger and larger Grievers as we traveled the mines, this made sense.
The symbol that Samus traces her hand over in the dirt is the one that appears when the guardian of a key dies. It stands to reason that the Grievers killed it. Green Energy reportedly originated from the volcano, which is a long way from here, but I guess there was a lot in the mines, too. It’s probably thanks to all the machinery and lava that we didn’t see any Grievers over there. Instead, we just get pretty much nothing other than Grievers over here. What’s their food source to sustain such a population?
In terms of boss difficulty, the Omega Griever is roughly on par with the Omega Pirate; I don’t think I beat that one first try on my first playthrough, either. However, this one feels quite a lot more unfair, because you can’t lock onto any points you can actually shoot. Yes, I know the game has been doing the free aim thing the whole time, but the areas you can hit get smaller as the fight continues until it’s surprising to land any hits at all when not using a Control Beam. I really thought the elemental charge shots would be more useful, thanks to the wide scattering of the shots, but I rather found it more helpful to simply not bother to lock onto the boss and definitely not bother with doing any charge shots until it was vulnerable. More hits is better than strong hits.