In 1944 an Australian Air Force member dug up some coins from a beach on the Wessel islands. They were kept in a tin for decades until eventually identified. Four were minted by the Dutch East India company, but five were from the Kilwa, a port city-state in modern day Tanzania.
Further exploration has found one more suspected Kilwa coin on another of the Wessel islands.
Kilwa started minting coins in the 11th century, but only two others had previously been found outside its borders: one at Great Zimbabwe, and another in Oman, both of which had significant trade links with Kilwa.
What other artefacts have been discovered in unexpected places?
Edit: A lot of great examples being discussed, but general reminder that extraordinary claims require extraordinary evidence. Take everything with a pinch of salt, particularly since a couple of these seem to have more ordinary explanations or are outright hoaxes.
Source: reddit post