Out Of Place Artifact, or OOPA, or OOPart, is an Artifact found in a context that defies conventional notions or expectations in regards to human history.  The topic is somewhat controversial, since some OOParts have been hoaxes, and others may be wishful misinterpretations, religious dogma, or honest mistakes.
This page seeks to list artifacts that seem to be anachronistic, or which modern science has had difficulty explaining. Items which have been proven as hoaxes, as well as items which lead to revisions in accepted history, or which were once baffling but now have mundane answers, can all generate numerous interesting ideas for use in RPGs. At least some of these aren't really OOPAs, but this is the closest category to which they belong.
- The Acambaro figures, from Acámbaro, Mexico, some of which are in the apparent form of dinosaurs.
- The Antikythera mechanism, a geared device manufactured ca. 100 BC, believed to be an orrery for predicting the motion of the sun, moon and planets.
- The Baghdad Battery, the name given to three terracotta jars, thought by some to be galvanic cells dating from the Sassanid dynasty (224-640 AD).
- The Baigong Pipes, pipelike features found in a cave in China.
- The Coso artifact, a lump of rock or clay containing a spark plug from the 1920s, though it allegedly took thousands of years to form.
- The Crystal skulls, claimed to have been found at Lubaantun, in Yucatan and in Belize.
- The Dorchester Pot, a Victorian-era candlestick found in Massachusetts, apparently alleged to pre-date European settlement in the Americas.
- The Dendra Lamps, representations of lotus flowers engraved into a relief in a temple dedicated to Hathor, Egyptian Goddess of the Milky Way, and alleged by some to actually represent electrical lamps.
- The Fuente Magna, discovered in Bolivia. Ceramic bowl with writing in alleged Sumerian cuneiform.
- The Glozel Tablets, discovered in France in the 1920s and 1930s, some of which were inscribed with an unknown, undeciphered alphabet.
- The Ica stones, from Peru, allegedly depicting anachronistic images such as dinosaurs and modern medical procedures.
- The Iron Man (Eiserne Mann), dating to the 13th century.
- The Iron pillar in India, dating around to AD 423.
- The Kensington Runestone, purported to be a 14th century Norse artifact found in Minnesota.
- The Kingoodie hammer, from Scotland, purportedly an iron nail dated from 460 to 360 million years ago.
- The Klerksdorp Spheres, from South Africa, dated 2.8 billion years ago – their regular shapes lead to claims that they were artificially created.
- The Lake Winnipesaukee mystery stone.
- The Maine Penny, found in Blue Hill, Maine. An 11th century Norse coin found in an American Indian shell midden. Over 20,000 objects were found over a 15-year period at the Goddard site in Blue Hill. The sole non-Native artifact was the coin. One hypothesis is that it may have been brought to the site from a Viking settlement in Newfoundland by seagoing Native Americans.
- The Piri Reis Map, a 16th Century map depicting the coastline of Antarctica as it appeared 6,000 years prior.
- The Spirit Pond runestones, claimed, like the Kensington runestone, to be from the 11th century or 14th century, found in Maine.
- The Swiss Watch found in a 400-year old Chinese Tomb
- The Tecaxic-Calixtlahuaca head, a terracotta head found in Mexico that some say is of Roman origin.
- The Wolfsegg Iron, a cubical block of metal in coal found in Austria.
If one envisions human technological advancement as being roughly parallel to the expansion and decline of human civilizations — that is, progressing in a "three steps forward, two steps back" sort of manner — then at least some (perhaps even many) apparent "anachronisms" are to be expected. A good example of this would be concrete, being used in the past by various ancient cultures only to be forgotten about and then re-invented at a later time by another culture, until the present, at which point the technology is employed globally and unlikely to slip into obscurity again without major upheaval. 
- Ancient Astronauts
- Charles Fort
- Reality Quake - Out Of Place Artifacts might have been generated by those
On a related note, the World Without Us website and book document how fast various artifacts of our culture would degrade and disappear once we're gone. It may prove useful to GMs seeking to create an alternate history or secret history setting that could serve as where one or more of the above OOPAs came from.
Game and Story Use
- Any of the above could serve as a MacGuffin in a pulp game with archaeologists.
- Could be evidence of sloppy work by lazy Time Travelers. They didn't clean up after themselves.
- Maybe one of these items bears a striking resemblance to some prop currently in the PC's possession? I see a Portal To The Past in your future.
- History could be radically different from what we imagine. Any of the above OOPAs could suggest a jumping off point for an Alternate History game.
- An NPC Con Man might try to sell an OOPA to the PCs. The players, assuming it's a genuine clue to a lost civilization, or possibly even a magic item, go to great lengths to acquire the object for their characters. Further study reveals it for a hoax, and the players discover they're on a different type of adventure than they'd previously imagined.