Hadrian's Villa has unveiled its latest secret, a monumental staircase complete with huge columns and a giant sphinx .

Archaeologists said the stairway, found in an area known as the Gymnasium, was probably the original entrance to the sprawling complex .

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

"These are extraordinary finds," said the archaeologist who made the discoveries, Zaccaria Mari .

"The coloured marble on the columns is simply superb while the sphinx is an amazing work." Mari 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 .

What's more, Mari went on, the new dig may have more secrets in store .

"We've just started here. The digging gets under way again on Tuesday." Hadrian's Villa, a few miles north of Rome at Tivoli, was the largest and richest Imperial Roman villa ever built .

Starting from his investiture in 117 AD, it took ten years to build with Hadrian himself showing his architectural skills as he paid homage to the most beautiful buildings in his Empire .

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 vast site - at least the size of Pompeii - was looted by barbarians and ransacked 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 fill the museums of Europe .

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