Onyx Mountain Caverns

King Cave - Boiling Springs Cave - Onyx Cave

Useful Information

Location: Newburg, 20 km west of Rolla.
I44, exit 172 3.2 km on North outer road, or exit 169 1.6 km on North outer road, Rd 269, after 1.4 km right 400 m.
(37.885764, -92.031230)
Open: closed.
Fee: closed.
Classification: SpeleologyKarst cave
Light: LightIncandescent Electric Light System
Guided tours: D=45 min.
Bibliography: Mary Catherine Smith (1998): National Register of Historic Places Inventory Nomination Form: Onyx Cave, Missouri Department of Natural Resources. pdf
Address: Onyx Mountain Caverns, Rt. 2, Box 549, Newburg, MO 65550, Tel. +1-573-762-3341
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
Please check rates and details directly with the companies in question if you need more recent info.


1892 2.4 m x 3.9 m shaft sunk to mine the onyx.
1990 opened to the public.
1999 listed on the National Register of Historic Places.
JUN-2006 closed to the public.


Onyx Mountain Caverns is a former show cave, which was purchased by the U.S. Forest Service. The cave was operated by Agnes and Harry Thiltgen until Harry died in 2006. At this time Agnes was in her 80s and "didn’t want to monkey with it anymore". The Forest Service installed new gates to protect the cave and transformed it into a place for endangered and threatened species. The cave is home to the Indiana bat and the gray bat. It will not be operated commercially anymore.

Onyx Mountain Caverns is a large and beautiful cavern. The huge entrance portal is followed by a cavern with a size of almost half an hectare, 120 m long, 30 m wide and 12 m high. It was once used by Woodland Indians as a shelter, some of their Flint artifacts are on display in the cave entrance. There is evidence of ceremonial use as well as of residential habitation. The huge entrance could have accommodated many small tribes of the area. The cave was used as a shelter for about 10,000 years, but the Woodland Indians had vanished when the first settlers came, so there is no historic description.

The floor of the cave shows some bear beds, shallow holes dug by bears to sleep inside. They spent the winter in the cave for hibernation. The tour also shows the cave river.

The speleothems of this cave have been mined during the Victorian era. As flowstone is locally called onyx, the company who did this was called the Imperial Onyx Company. Several carloads were removed and shipped to St. Louis. When cut, flowstone has a structure similar to marble or onyx. It was used in contemporary Victorian-style homes and public buildings for decorative stone. Obviously flowstone is neither marble nor onyx. Onyx is a variety of quartz, flowstone is calcite, marble is calcite but it is formed during a metamorphic process which is very hot with high pressures. Flowstone is formed in a cool and wet cave with atmospheric pressure. The rock is not very suitable as decorative stone, as it often contains holes and the mining is too difficult, so this era fortunately ended soon, and most flowstone remains in the cave.

Another intended use of the cave as a tuberculosis hospital in the late 19th century was fortunately never realized. The cool and damp air would probably have caused a lot of deaths from pneumonia instead of curing tuberculosis. Speleotherapy is only working on asthma, not on infections. In the 1950s the next threat came, the entrance hall was intended as a jet propulsion lab for testing jet engines for military fighter planes. Fortunately it was never realized.

The cave was visited for a very long time. At the turn of the century, tourists coming from St. Louis on the Frisco railroad were brought up river from Jerome. But only the large entrance room of the cavern was visited. The cave was finally developed and opened to the public in 1990.

The cave has still some fine speleothems, despite the onyx mining. There is a 10 m high flowstone and numerous stalactites and stalagmites. There are also draperies and soda straws. The most exceptional speleothems are box work formations. An endemic algae, found only in this cave, is only of scientific interest. It is published in the National Sciences Annual.