The Coso artifact is an object claimed by its discoverers to be a spark plug found encased in a lump of hard clay or rock on February 13, 1961, by Wallace Lane, Virginia Maxey and Mike Mikesell while they were prospecting for geodes near the town of Olancha, California, and long claimed as an example of an out-of-place artifact.
A spark plug encased in a 500,000-year-old "geode" would represent a substantial scientific and historical anomaly, as spark plugs were invented in the 19th century. Critics say the stone matrix containing the artifact is not a geode but concretion that can be explained by natural processes that can take place over decades or years.
Following its collection, Mikesell destroyed a diamond-edged blade cutting through the matrix and discovered the item. In a letter written to Desert Magazine of Outdoor Southwest a reader stated that a trained geologist had dated the nodule as at least 500,000 years old. The identity of the geologist and means of dating were never clarified, nor were the findings ever published in any known periodical. Furthermore, at the time of the artifact's reported discovery there was no method for dating the concretion.  There are examples of accretions surrounding iron or steel artifacts, some of them discussed by J. M. Cronyn. 
Criticism and analysis
- An ancient advanced civilization (such as Atlantis);
- Prehistoric extraterrestrial visitors to Earth;
- Human time-travellers from the future leaving or losing the artifact during a visit to the past.
An investigation by Pierre Stromberg and Paul Heinrich, with the help of members of the Spark Plug Collectors of America, identified the artifact as a 1920s-era Champion spark plug, widely used in the Ford Model T and Model A engines. SPCA President Chad Windham and other collectors concurred with his assessment.
Stromberg and Heinrich's report indicates the spark plug became encased in a concretion composed of iron derived from the rusting spark plug. Iron and steel artifacts rapidly form iron-oxide concretions as they rust in the ground.
The location of the Coso artifact is unknown as of 2008. Of its discoverers, Lane has died, Maxey is alive but avoids public comment, and Mikesell's whereabouts are not known.
- Stromberg, P., and P. V. Heinrich, 2004, The Coso Artifact Mystery from the Depths of Time?, Reports of the National Center for Science Education, v. 24, no. 2, pp. 26–30 (March/April 2004).
- Maxey, V., 1961, The Coso Geode... in Letters...From Our Readers. Desert Magazine of Outdoor Southwest. v. 25, no. 5, p. 4. (May 1961).
- Walker, M. (2005) Quaternary Dating methods Wiley, New York, New York. 304 pp. ISBN 978-0470869277.
- Cronyn, J. M. (1990) The Elements of Archaeological Conservation Routledge, New York, New York. 326 pp. ISBN 978-0415012072.
- Andrew O'Hehir (August 31, 2005). "Archaeology from the dark side". Salon.