A massive monumental platform of unknown purpose has been discovered in the ancient site of Petra.

The raised area, which measures about 56 metres (182ft) by 49 metres (159ft), appears to be unlike any other feature in the surrounding area, researchers say.

It was found using a combination of satellite pictures and images obtained from drones.

The study that discovered it, published in the Bulletin of the American Schools of Oriental Research, says it suggests there may be many more ancient features yet to be uncovered.

The study's authors, Sarah Parcak and Christopher Tuttle, write: "Petra, Jordan, has one of the richest histories of archaeological exploration in the world, as a World Heritage site, visited yearly by half a million tourists.

"Archaeologists have documented thousands of carved and constructed monuments within the ancient city centre as well as in the wadis (river valleys) and mountains that surround it.

"Even after two centuries of of fieldwork in Petra and its environs, new discoveries and identifications of monumental structures continue to be made both within and around the urban centre."

Petra was a major centre on the ancient trade routes that supplied the Roman Empire with sought-after goods from Persia and Arabia, and rivalled Palmyra for power and influence.

But its origins are older and remain a mystery, with much speculation remaining over the identity of the group who ruled the site - the Nabataeans.

Some scholars say the city features in ancient Latin and Greek texts and may even have been referred to in the oldest books of the Bible.

National Geographic, which first reported on Ms Parcak and Mr Tuttle's paper, said the monument is roughly as long as an Olympic-size swimming pool and twice as wide.

It was found only about 800 metres (half a mile) south of the ancient city's centre and could have had a public, ceremonial function, the pair believe.

The main platform encloses another slightly smaller platform that would have originally been paved with flagstones, with a row of columns on its east side, at the top of a huge staircase.

The paper's authors speculate that, because of the finding of pottery from the second century BC, the platform was probably built at the same time or slightly earlier than many of Petra's other public monuments.