Watch closely during the scene after the cave-in. While the girls are arguing about how to get out, Sarah is moving her flashlight about the chamber. She shines it on a ledge for a fraction of a second as she is moving the light around, and you can can see the shadow or outline of a crawler. When she passes by the ledge again with the flashlight, the shadow is gone, and you can faintly hear a scurrying sound as it is running away.
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