While on a helicopter flight off the coast of Tanzania, a low tide allowed a scuba diver to spot an unusually-shaped formation in the water.
Archaeologist believe what they found was an ancient sunken city, known as Rhapta which thrived 2,000 years ago.
Rhapta is believed to be Africa’s first metropolis and a trading hub for tortoiseshell and metal weapons.
But little is known about Rhapta’s story since its disappearance more than 1,600 years ago.
Its location has not yet been firmly identified, although there are a number of plausible candidate sites off the coast of Tanzania.
Now Alan Sutton, a scuba-diver, might have come across the ancient city during a series of dives.
‘The formation was quite far in the distance and I took a photograph and then blew it up,’ Mr Sutton wrote in a blog post about the discovery.
‘After several unsuccessful attempts to find the formations due to low water visibility…we managed to find them on a spring low tide,’ Mr Sutton said.
‘What we found was far larger than expected. A series of what appear to be wide foundations ring a large area.
‘Along the entire perimeter created by these foundations, many thousands of square and oblong blocks lie to either side. Some have fallen right off the foundation and others are still leaning against it.’
A professor of archaeology later visited the site and told MailOnline he thinks it could be the lost city.
‘I went there only on 20th of May,’ Professor Felix Chami, archaeology professor at Dar es Salaam University, Tanzania, told MailOnline.
‘Truly the ruins seem ancient of probably Roman times.’
‘It could be the metropolis of Rhapta as reported by Claudia Ptolemy of the 2nd century CE,’ he said.
‘We are looking for artefacts of he period probably now underwater to confirm that it is Rhapta.’
The city of Rhapta is mentioned by Claudius Ptolemy the Greco-Egyptian writer, in his work ‘Geography’, dating back to 50 AD.
His text describes an account of Diogenes, a seaman on the India trade, blown off course who visited Rhapta.
A German explorer also visited Mafia Island in the early 1890s and mapped the island. The map mentions the Portuguese fort as having been washed away by the sea.
‘For several years we have been looking for the fort in this area without success,’ Mr Sutton said.
‘Its location is not questionable if one relies on the Roman document, put at Lat. 7.7 south and at the sailable river,’ added Professor Chami.
‘The ruins are now on the bay of the River known by the Romans as Rhapta bay.
‘The island on which it is attached to is Mafia also known by Ptolemy as Mafiaco.’
‘Exactly whether this is Rhapta remains ‘a puzzle for the archaeologists to figure out,’ Mr Sutton said.
‘It seems very old and to have been extremely well constructed, in a fashion unlike the architecture of other ruins in Tanzania and doubtless the site will keep archaeologists busy for many years.
‘Without a large amount of research it is impossible to say exactly what the site is. It however appears to be a very old harbour city.’