Roof tiles for use at Edo Castle are among relics found at an underwater archaeological site off Hatsushima island in Atami, Shizuoka Prefecture. (Kazuhiko Okada)

ATAMI, Shizuoka Prefecture--Roof tiles for Edo Castle, including a gargoyle tile featuring the Tokugawa shogun clan’s coat of arms, were among well-preserved items found from a ship that sank more than 300 years ago.

The Asian Research Institute of Underwater Archaeology (ARIUA), a Fukuoka-based nonprofit organization, held a study session from Dec. 13 through 16 at the site where a cargo ship sank on a voyage from Osaka to Edo, or present-day Tokyo.

With cooperation from the Hatsushima Diving Center, 11 people, including researchers and students who are ARIUA members, dived to a 20-meter-deep seabed off the western coast of Hatsushima island.

The study was subsidized by The Asahi Shimbun Foundation.

In the clear seawater with temperatures just under 20 degrees, a black mass came into view as an Asahi Shimbun reporter began the descent.

A closer look revealed countless numbers of roof tiles arranged in an orderly manner and piled up in many tiers.

Most of them were flat tiles, but there were also round tiles, flat eave tiles and round eave tiles.

A wild ginger trefoil coat of arms of the Tokugawa clan was embossed in bold relief on a ridge-end gargoyle tile in the sandy soil.

Earthenware mortars, whetstones and residual wood from the hull were also found at the underwater archaeological site, which extends 5 meters per side.

The tiles were made between the 17th century and the early 18th century by Osaka’s Terajima family, the exclusive manufacturer of Edo Castle’s roof tiles.

The mortars were produced in Tanba province, straddling today’s central Kyoto Prefecture and eastern Hyogo Prefecture.

The underwater site is still not eligible for legal protection.

The ARIUA hopes to reveal more details of the study, which began in 2011, so the location will be registered as a “site containing a buried cultural property” as defined in the Law on Protection of Cultural Properties.

Toshiaki Hayashibara, a 58-year-old part-time lecturer of underwater archaeology with Tokyo University of Marine Science and Technology, leads the study team and sits on the ARIUA board of directors.

“This underwater site is valuable because it has things that can tell us about the Tokugawa Shogunate and Edo Castle, and even about industries and distribution systems of the time,” Hayashibara said. “Our study is important, not the least for the site’s protection and its use in education and other purposes.”