Coso Artifact in a range of hills outside Olancha California where they owned the enterprise "LM & V Rockhounds Gem and Gift Shop". They were out trying to augment their stock and threw a number of lumps into their rock-bag for processing back in their workshop. They were particularly interested in geodes [nice examples L] which are hollow geological concretions often crappy on the outside but with nice bright colourful crystal interiors: they make pretty, useless, ornaments and formed the backbone of LM&V's business. When this potential geode was cut in half with a diamond saw it turned out not to be hollow and "dang near busted the diamond saw" [I paraphrase]. In the middle of the concretion, which included sea-shells estimated to be half-a-million years old, was something that looked like a spark-plug. It had a magnetic core surrounded by white ceramic and bits of copper, was roughly hexagonal in shape and was the size of a spark-plug. It caused a bit of stir when the Coso Three wrote up a report in the local Outdoors magazine. I can't show you a picture, however, because the Coso Artifact has disappeared in the intervening 55 years. I could show you pictures, including X-rays, that were supposed to be taken in the 60s, but I decline to do so. The Antikythera Mechanism is still there for our investigation.
If you take an assertion by an unnamed geologist that the exterior is 500,000 years old and believe that the object inside is a spark-plug then we have a problem. It implies that someone was dropping spark-plugs in the wilderness at a time when all the other surviving man-made artefacts are chipped stone tools. That seems unlikely, so that leaves Christopher Lloyd doing some repairs in Back To The Future XV or gasoline-powered space-ships driven by Little Green Men. Both of these explanations have been proposed and supported by people who have difficulty integrating and evaluating evidence. I think we can take a cue from Tony Kavanagh's considered dismissal of a nonsense story published in Nature: "Extraordinary findings require extraordinary levels of proof". Here we don't even have the primary evidence any more. But that hasn't stopped the The Spark Plug Collectors of America asserting the age and brand (Champion) of the said spark-plug. Only in America: who would consume their leisure hours collecting spark-plugs when they could be buying a nice geode to put in the middle of their coffee table? But don't take my assessment: 400,000 people have watched this youtube clip.