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It seems to be that the traps, which the Stalkers refer to colloquially as Bug Traps, while the scientists who study The Zone (from a safe distance) call them gravicoconcentrates, a name that describes the phenomenon and gives them some false sense of understanding it, and basically they are areas of super high gravity contained impossibly within certain boundaries. Anything that enters it gets literally squashed flat like in a cartoon (or that's how I envisage it) and can sometimes be seen there for years afterwards. They test for these things by throwing the nuts and bolts.
There are plenty of other things in The Zone though that they have to watch out for... happy ghosts, the hell slime, and the grinder (which it seems you can only get past by tricking someone else into it and then walking through while it's grinding them).
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