What were ancient Roman coins doing underneath a Japanese castle?

Roman coins aren’t exactly an uncommon find at many archeological digs in the Mediterranean world, but this week’s discovery beneath the ruins of a Japanese castle marks the first time that Roman coins have been discovered in Japan.

The ten copper coins, four copper pieces between 1.6 and two centimeters in diameter, were found beneath Katsuren castle in Okinawa, Japan's southernmost prefecture, and the site of controversial US military bases.

Katsuren was built around the 13th century, and designated a UNESCO World Heritage site in 2000. 

Four of the coins are believed to be from Rome in the third or fourth centuries. Other finds at the site include pottery from the same geological layer as the Roman coins, which dates to the 14th or 15th century, a coin from the 17th-century Ottoman Empire, and five other coins of unknown origin that are currently under analysis. 

The local education board commented on the find, saying that these coins are “precious historical material suggesting a link between Okinawa and the Western world.”

That link, however is likely an indirect one. Archaeologists are still not sure precisely where the coins came from, although it is likely that they arrived between the 14th and 15th centuries, when Okinawa's Kingdom of Ryukyu had a thriving maritime trade with other Asian nations that had contact with the Roman empire.

Some scholars are skeptical of the find, however, especially since the suspected 17th-century Ottoman coin likely arrived so much later. The feudal lord who once lived in the castle was known to deal in regional trade, but researchers have little evidence to link him to greater international trade. 

"Roman coins were also found in Southeast Asia, so the coins are likely to have arrived in Okinawa through trading with Southeast Asia," Yasuhiro Yokkaichi, a researcher at Waseda University’s Institute for Central Eurasian History and Culture, told the Asahi Shimbun. "I want those who conduct an investigation of this discovery to closely examine different possibilities including that the coins came to be mixed in after much time had passed."

Roman coin caches are far from unheard of. Last fall, a Swiss farmer unearthed more than 4,000 coins in his orchard, the largest trove ever found outside Britain, and in 2010, a British treasure hunter found 52,000 coins in a field. Never before, however, have Roman coins of this vintage been discovered in Japan.

Roman coins are also unlikely to have been in circulation among merchants at the time, as many used Chinese coins, archaeologist Hiroki Miyagi told Agence France-Presse.

"East Asian merchants in the 14 and 15th centuries mainly used Chinese currency, a round coin with a square hole in the middle," said Dr. Miyagi, "so it is unlikely that the Western coins were used as a means of currency."

Initially, Miyagi thought that the coins were perhaps dropped by US soldiers during World War II. However, after washing them off, it became clear that they were something more. X-ray analysis of the coins revealed that they had Roman images, including what is possible a portrait of Emperor Constantine, AFP reports. 

Researchers say that the next step is to determine how the coins ended up in Japan.

"The Chinese ceramics and coins that we found date back at least 600 to 700 years and we'd like to analyze those objects in tandem with these coins to work out how the coins may have ended up here," Miyagi told CNN.