The Coso artifact, a 500,000 Year old object is 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.
If a spark plug is encased in a 500,000-year-old “geode,” this finding would represent a substantial scientific and historical anomaly, as spark plugs were invented in the 19th century. Critics have argued, however, that the concretion, not geode, containing the Coso artifact can be explained by known natural processes and credible evidence for it being 500,000 years old is completely lacking.
Some theories of the origin of this out of place artifact include:
- 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.