The Hell Creek Formation is a geological formation located in the northwestern United States, primarily in the states of Montana, North Dakota, and South Dakota. It dates back to the late Cretaceous period, around 66 million years ago, and is known for its rich fossil deposits, including dinosaur remains and ancient plants. This formation is particularly famous for being one of the last places where dinosaurs lived before the mass extinction event. Fossils of well-known dinosaurs like Tyrannosaurus rex and Triceratops have been discovered here, making it a significant site for paleontological research and understanding prehistoric life.