OoPArts, or Out of Place Artefacts, are artifacts "out of context", that is, inconsistent with the archaeological context in which they were found or the time they are supposed to belong to. Some are authentic objects, unique in their characteristics, witnesses of an isolated genius like the Antikythera mechanism. But most of the objects presented as OoPArts in countless pseudo-archeology books or websites are either fakes, or fanciful interpretations of real objects. In this section, we will describe some cases of OoPArts used as "evidence" of forgotten technologically advanced civilizations, or of past visits of aliens on Earth.