Roman coins at a Japanese castle's ruins startled archaeologists who unearthed them on Wednesday because it was the first time anything like them had been discovered there.
The 10 bronze and copper coins -- the oldest dating from about 300-400 AD -- were discovered at Katsuren castle in southern Okinawa, said Agence France-Presse.
"At first I thought they were one cent coins dropped by U.S. soldiers," archaeologist Hiroki Miyagi told AFP, referring to the cluster of American military bases and thousands of troops in the area. "But after washing them in water I realised they were much older. I was really shocked."
A team of researchers have been excavating the castle, which is a UNESCO world heritage site, since 2013.
An X-ray analysis of the dime-sized coins showed some were embossed with Roman letters and possibly the image of Emperor Constantine I and a soldier holding a spear.
Several others dated from a later period -- the 17th century Ottoman empire.
Researchers were left scratching their heads about how the coins ended up at the castle, which was built sometime in late 13th or early 14th century and abandoned about 200 years later.
It was once the residence of a feudal lord, whose wealth was linked to regional trade but he was not known to have had business ties with Europe.
"East Asian merchants in the 14 and 15th centuries mainly used Chinese currency, a round coin with a square hole in the middle, so it is unlikely that the Western coins were used as a means of currency," said Miyagi, who also teaches at Okinawa International University.
"I believe they probably got the coins in Southeast Asia or China."