Coso artifact

The Coso artifact is an object claimed by its discoverers to be a spark plug encased in a geode. Discovered on February 13, 1961, by Wallace Lane, Virginia Maxey, and Mike Mikesell while they were prospecting for geodes near the town of Olancha, California, it has long been claimed as an example of an out-of-place artifact. The artifact has been identified as a 1920s-era Champion spark plug.

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 that the stone matrix containing the artifact is not a geode but a concretion that can be explained by natural processes that can take place over decades or years.