The remains of a dinosaur found in the Hell Creek Formation of North Dakota are so well preserved that some scientists are just “gobsmacked.”

For decades, museums and textbooks confidently asserted that mud rocks—such as limestone, siltstone, mudstone, and shale—were formed over vast eons as super-fine sediments slowly settled to the bottom of shallow lakes or seas.

In 1980, a theory was proposed that an asteroid or comet impact was primarily responsible for the mass dinosaur extinctions that were observed in the fossil record. But while the impact tale has become widely accepted in popular culture, critical questions remain unanswered.