[Thulorongil]

I agree, Sharkû. All the igneous activity, landslides, and whatever else might logically have been going on in the area would make the landscape almost constantly change. Why spend time constantly clearing up over there when Sauron could be training his armies, building new wings onto his fortress, planning attacks, or gloating? Especially when there was almost no chance at all in his mind of someone using the area to attack him? Also, why spend time cleaning it up when he could use it, like Theron said, to even more thoroughly discourage anyone and everyone. So many things that the "bad guys" in Tolkien's writing do are based on frightening and intimidating the enemy. (And, come to think of it, frightening and intimidating their allies, too.)