The Coso Artifact is an OOPART (Out Of Place ARTefact) that was unearthed in February 1961 in the Coso Mountains at a spot near Olancha, California. The find was made by a trio of mineral hunters named Mike Mikesell, Wallace Lane and Virginia Maxey who between them ran the LM & V Rockhounds Gem and Gift Shop in Olancha. The artefact (sorry, I'm British) was found inside one of a number of suspected geodes that the team picked up on the slopes of a 4,300ft hillside near Owens Lake, west of Death Valley. Attempting to cut the 'geode' open the next day, Mikesell found he had sawed across a circular plug of hard white material resembling porcelain which had a magnetic metal centre. An X-ray of the find revealed that the object was indeed a spark plug.

A large part of the reason why the Coso Artifact continues to attract so much attention, and so many outlandish claims (some by creationists, others by proponents of the so-called 'Ancient Astronaut' theory, suggesting that earth was visited in the distant past by advanced races who, in many tellings, had a lot to do with kick-starting human civilisation) is that it is very poorly recorded; no one has seen the object for years, and no one knows where it currently is, so it can't be subjected to any sort of re-examination. What we do have is a rather sketchy account of its provenance, some photos and an X-ray.

There are a few key points to make about the Coso Artifact and its provenance:

• The 'rock' the object was found in was not a geode, and hence it was not discovered inside a geological feature that – so the finders claimed they were told by an un-named geologist – had formed at least 500,000 years earlier. Archaeologist Keith Fitzpatrick-Matthews notes that "geodes have very precise characteristics – a thin outer shell of dense chalcedonic silica, and a layer of quartz crystals internally – which the 'Coso Artifact’ does not possess." Rather, the "rock" had a hardness of only 3 on the Moh scale of mineral hardness, which is not consistent with a geode but is consistent with hardened clay. This seems to tie in to Virginia Maxey's suggestion that it might have been "something that lay in a mud bed, then got baked and hardened by the sun" (though she went on to add: "...Or else it is an instrument as old as legendary Mu or Atlantis.”)

• The spark plug was not the only object found in the same spot. The same sample of clay material also contained a nail and a washer.

• Chad Windham, President of the Spark Plug Collectors of America, and three other spark plug collectors, have independently identified the Artifact as a recognisably human design: it is a 1920s Champion spark plug of the sort ubiquitously used to turn the engines of Model T Fords.

Most likely, therefore, the Coso Artifact is part of a small collection of junk dropped on the spot some time in the 1920s, possibly shed by the owner of a Model T used in mining operations in the area.

Now, as to whether the Artifact was an example of a hoax or bad archaeology, the available evidence points to the latter. There seems to be nothing to suggest that the object was hoaxed or planted by the finders, though that might certainly have been possible; on the other hand, they did little to de-mystify the find:

• The only person permitted to make a study of the Artifact was Ron Calais, a creationist.

• The name of the "geologist" who claimed the object was half a million years old has never been released.

• The finders attempted unsuccessfully to sell the find for $25,000.

• The fact that Virginia Maxey was familiar with the concepts of Atlantis and Mu suggests that at least one of the finders had what might charitably be described as a 'bad archaeology' mind-set.

All in all, the only people who could take the artefact seriously as a real mystery would be people who wanted to believe in it. The fact that it's still regularly cited as an unexplained find shows that there are plenty of them around.