Sauron and some of the orcs had survived...........
after the destruction of The One Ring in the lake of fire. I specifically recall it being mentioned in the book. Since Sauron was an immortal spirit, the destruction of the ring wouldn't erase him out of existence. He'd become a small embittered cloud floating around in the tunnels far below ground. Those orcs that survived the final battle of Mordor would flee deep into the underground as well and would always remain there.
It would have been interesting if Tolkien had wrote a sequel to LOTR and had the story set 2 or 3 thousand years later. A band of men would find the secret tunnels to the deep depths below and go down to explore, unaware of the dangers that lurked in the shadows. Just a thought.
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