I watched this when I was in elementary school (maybe in 2003), and couldn't sleep well for days. I would really like to know the name of the movie. From what I remember: In the beginning a boy played around with a girl's necklace and it accidentally fell into the sewage system. Then they decided to go into the sewage system to search for it, leading them to an underground world where a monster lived. The monster ate or killed humans and walked around an elevator (not sure about this part). There was a scene when the monster cannot touch the kids because they circled themselves with some magical item. The teenagers tried to find the way out. Hope someone has seen this movie~~
As per the comments above, it's likely that you're looking for Demon in the Bottle (1996). It features four teenagers who accidentally release a demon from its bottle. He drops the necklace around the 13 minute mark after retrieving it from a bully.
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