Hadrian's Villa promises to give up more secrets after a monumental staircase and giant sphinx were found last week, archaeologists said on Tuesday .

"This is one of the least-known spots in the area and could well produce more finds," said the head of the new dig, Zaccaria Mari .

Archaeologists are hoping the huge marble staircase could be lined with sphinxes as was the custom in Ancient Egypt .

They said the ornamental motifs might be buried further down and thus in better condition than the sphinx unearthed last Friday, which had lost its head .

Hadrian's Villa, a few miles north of Rome at Tivoli, was the largest and richest Imperial Roman villa ever built .

Started soon after Hadrian's investiture in 117 AD, it took ten years to build and the emperor himself showed his architectural skills in paying homage to the most beautiful buildings in his Empire .

Talking about the prospect of more finds, Lazio archaeological superintendent Anna Maria Moretti admitted that the immediately surrounding area had been comprehensively pillaged by 16th-century popes and cardinals. "But you never know what might still be down there". "Already, this latest discovery has enabled us to show the site had a different and nobler use" .

The stairway and sphinx were found in an area dotted with seven buildings and collectively called - mistakenly as it now turns out - the Palestra (Gymnasium). Experts now think it was probably the original entrance to the sprawling complex .

Moretti said visitors would be allowed in to have a peek at the dig in "about a year," getting their first glimpse of the staircase, sphinx, and anything else that turns up in the meantime .

A statue of an athlete and a huge theatrical mask, both in marble, have also been found at the site .

The finds are the most exciting things to come out of the villa in years. "The coloured marble on the columns is simply superb while the sphinx is an amazing work," Mari said when they came out of the ground .

He said the 8.5m wide staircase and the statuary were probably made around the end of the villa's construction, towards 130 AD, but the 2.5m-long sphinx might be even older .

"We think it came from one of the imperial workshops but there's a chance it might have been brought back from Egypt," he said .

Protected by a beautiful park, Hadrian's Villa is one of the most evocative classical sites in Italy and draws thousands of visitors a year .

One of the best-preserved parts is a recreation of the famous statue-lined pool shrine at Canopus in Egypt - one of many memorials to the emperor's boy-lover Antinoos .

The architectural gems were linked by pathways and passages - including a subterranean one inspired by a classical description of the Underworld - to form a sort of small city, used by Hadrian as a summer court .

The vast site - at least the size of Pompeii - was looted by barbarians and plundered by later stone-hunters but has still disgorged hundreds of artistic treasures since the first excavations in the 16th century .

The almost 300 art works discovered there are scattered around the museums of Europe .