Cliff Cave formed as rainwater and melting snow soaked into cracks and pores in limestone, mixed with carbon dioxide to create carbonic acid, and slowly widened crevices into passages over as long as 100,000 years. Since prehistoric times, caves have provided shelter, and caves in Missouri and around the world have yielded drawings, pottery vessels, organic material, stone tools, and weapons from early civilizations. Cliff Cave remains a steady 57 degrees Fahrenheit year-round, and in the mid-1800s the Cliff Cave Wine Company used it as a wine cellar and built the rock walls at its entrance. Five of Missouri's fifteen bat species are known to live inside Cliff Cave, including the federally endangered Gray Bat and Indiana Bat; they hibernate there in winter, nest in trees in the park and wooded areas along the Mississippi River during the rest of the year, and help control insect populations. Public access is limited to protect the endangered bats and to reduce danger from flash flooding. The cave was also called Indian Cave because Native Americans once used it for shelter and storage, and chert rock tool fragments embedded in the clay floor at the entrance show Native Americans occupied the area before the 1600s. Osage people living in the area used the cave for centuries, taking shelter near the front and using deeper areas for ceremonies; with its high entrance ceilings, spacious dry floor, nearby spring, and location near but above the river's flood levels, it offered refuge, and it remains a Sacred Site to the Osage Nation today.