Last month we broke the story of an amateur diver discovering a large swath of ruins off the coast of Mafia island: now we can tell you that an archaeologist is ‘cautiously optimistic’ the ruins are that of the ancient Roman city of Rhapta.

“I can’t relate the structures with any other building style. It’s not a Swahili settlement, it’s not Portuguese, it’s not German, it’s not British. There’s nothing else out there other than Rhapta,” says archaeologist Felix Chami from the University of Dar es Salaam.

“I think we can argue that we may have found Rhapta, pending further investigation,” says Chami, who visited the site over the weekend.

Chami says some pieces of ceramics as well as the 1919 wreck of a dhow carrying a load of roofing tiles were found but the challenge remains that the ‘very large settlement’ is largely underwater and will require marine archaeologists and extensive funding to explore it properly.

Diver Alan Sutton first spotted the mysterious ruins by chance as he passed by the area in a helicopter at low tide years ago and after several unsuccessful attempts, he was able to explore the 3.7 kilometre long ruins up close in April.

Rhapta was a first century CE trading town at the southernmost edge of the Roman empire, thought to have traded ivory and spices.

Roman artifacts have previously been found in the area around Mafia island.

Read more: Mafia ruins could date back to Roman Empire