Around 340 million years ago, a warm shallow sea covered most parts of what would later become Utah. As small marine animals died, their skeletons settled in the sediments. Going through cementation and compaction, the sediments hardened into limestone rock. Upon close inspection, the fossils of these ancient marine animals are now visible in the layers of rock. These fossils can be seen near the entrance to Hansen Cave and inside the cave. The most common types of fossils are rugose (or horn) coral, tabulate coral, crinoids, and brachiopods. These oddly shaped, white deposits in the rock serve as evidence of a constantly changing planet.