Cave-in-Rock State Park is an Illinois state park on 204 acres (0.83 km²) in Hardin County, Illinois in the United States. The state park contains the historic Cave-in-Rock, a landmark of the Ohio River. It is maintained by the Illinois Department of Natural Resources (IDNR).
During the 1790s and first decade of the 1800s, Cave-in-Rock reached the height of its notoriety. Flatboats carrying farm produce from Kentucky, Ohio, and southern Indiana began to float down the Ohio River towards the marketplace in New Orleans. As a known Ohio River landmark, the cave was a landmark of this dangerous journey. Al least twice during this 20-year period the cave became a hideout for notorious bandits that preyed upon the lawless river commerce.
Later in the 1800s, Cave-in-Rock was tamed by settlers who formed the river town of Cave-in-Rock, Illinois near the cave. The town survives to this day. It became the site of a river ferry, adjacent to the state park, that crosses the Ohio River from Illinois Route 1 to Kentucky Route 91 across the river.