Onyx Cave, near Newburg, Missouri, is a historic natural cave also known as King Cave and Boiling Springs Cave. Its principal chamber is broad and open rather than maze-like, measuring about 85 feet wide, 250 feet long, and 33 feet high, with two passageways extending from it. The cave is especially known for its translucent cave onyx, which ranges from white to cream and sometimes shows reddish bands, as well as for dripstone formations including stalactites, stalagmites, columns, and draperies. In 1892, a vertical shaft was sunk to mine the onyx, reflecting a brief commercial use, and the cave was listed on the National Register of Historic Places in 1999.