Of most concern is the fifth-century BC Parthenon temple, whose roof was destroyed during a 17th century siege of the Acropolis by Venetian forces.

"For centuries, rainwater could not penetrate the foundations," Manolis Korres, an architect with extensive experience of the Acropolis restoration effort told a recent gathering of Greece's state archaeological council.

"But in the wake of the roof's collapse, water has been seeping into the floor supports and wearing down the rocks...the surface is retreating," Korres said, according to a report in Eleftherotypia daily.

The threat of further damage has led experts to contemplate covering the Parthenon with a modern roof, Ta Nea daily said.

"We are closely following the problem," Acropolis site supervisor Alkistis Horemi told AFP. "Special machinery is used to monitor the walls surrounding the monument site at all times."

Test drilling carried out on four occasions has discovered cavities under and around the Parthenon, said Korres. The problem is exacerbated by the fact that an ancient drainage system built into the site no longer functions.

The Acropolis, a World Heritage site, has been undergoing restoration for over 20 years. The majority of these works are expected to be completed by late 2006.