The Coso artifact is an object incorrectly said by its discoverers to be a spark plug inside a geode. It was found on February 13, 1961, by Wallace Lane, Virginia Maxey, and Mike Mikesell while they were searching for geodes near Olancha, California. It has been claimed as an example of an artifact that does not fit with known history. The artifact is actually a 1920s-era Champion spark plug enclosed in a rock formation.

A spark plug inside a 500,000-year-old "geode" would be a major scientific and historical mystery, since spark plugs were invented in the 19th century. The rock surrounding the artifact is not a geode but a type of natural rock formation that can form over decades or years, not millions of years.

Discovery

After collecting the item, Mikesell broke a diamond-edged blade by cutting through the surrounding material and found an object. The object was a porcelain cylinder with a small metal rod that was later found to be magnetic. A letter to Desert Magazine of the Outdoor Southwest mentioned that a geologist estimated the nodule to be at least 500,000 years old. However, the geologist’s name and the method used to determine the age were not explained, and the results were never published in any known publication. At the time the artifact was reportedly discovered, there was no known way to date the concretion. Some examples of similar build-ups around iron or steel objects have been discussed by J. M. Cronyn.

Criticism and analysis

There are several unscientific theories about the artifact's origin, including the following:

An investigation by Pierre Stromberg and Paul Heinrich, using X-rays of the object and with help from members of the Spark Plug Collectors of America, identified the artifact as a 1920s-era Champion spark plug. This type of spark plug was commonly used in Ford Model T and Model A engines. SPCOA President Chad Windham and other collectors agreed with this finding.

Stromberg and Heinrich's report explains that the spark plug became surrounded by a hard, rock-like covering made of iron. This covering forms when iron and steel objects rust underground over time.

On April 12, 2018, Stromberg was contacted by the family of one of the people who originally found the artifact. They offered him the chance to examine the artifact in person. Stromberg accepted and also arranged for a geologist from the University of Washington Earth and Space Science department to inspect it. Both inspections confirmed the earlier conclusion that the artifact was a 1920s-era Champion spark plug.

It had been claimed that fossilized shells on the artifact's surface were 500,000 years old. However, the University of Washington geologist found no evidence to support this claim. This raises questions about the qualifications and abilities of the geologist who made the original claim in 1961. The artifact became famous because of the shell claim. As of 2019, the artifact is displayed at the Pacific Science Center in Seattle as part of an exhibition called "What Is Reality?"