In the past two and a half thousand years, the temples of the Acropolis have suffered fire, bombing and earthquake. Now, scientists are trying to save them from a new modern enemy: pollution.

Standing on a hilltop at the centre of Athens, a city of 4 million people, the Acropolis' elaborately sculptured stones have fallen prey to a film of black crust from car exhaust fumes, industrial pollution, acid rain and fires.

A team of Greek engineers and restorers are using an innovative laser technology system to clean the surface of the ancient monuments, uncovering colours and ornamentation hidden for decades.

"It is very serious," said Maria Ioannidou, director of the Acropolis Restoration Service, of the pollution. "It destroys sculptural, structural and painting details. One of our aims is to regain these cultural details using new technology."

For years the team tested 40 different methods, including mechanical and chemical processes, to find the safest solutions to restore the white of the marbles without losing detail.

The winner was the brainchild of Crete's Foundation for Research and Technology, which created a system that uses two laser beams of infrared and ultraviolet rays simultaneously.

These rays have been used separately to clean ancient marble, but it was found that one left a yellow tint while the other left a grey one. The new system blasts off layers of black film leaving the marble details intact, without discoloration.

But it is a risky process.

"If you remove something you cannot put it back in place, so we must be quite sure that we remove unwanted pollutants and leave ... all the information on the original surface," said Evi Papaconstantinou, the chemical engineer in charge of the team.

The system was first used on the sculptures of the west frieze of the Parthenon temple in 2004. Now the team has begun a second operation on the porch of the Caryatids, where besides pollution they must erase soot from fires and the mistakes of past restorers who tried to mend the roof with cement.

Scientists first scan the marbles with ultrasound and an infrared imaging and spectroscopy system to reveal what lies beneath the black crust. To their astonishment, they found colours, ornamentation and script that had been hidden for years.

Even wearing goggles, restorers can work only for two hours a day because of the flashing rays from the laser. They lie on a reclining doctor's chair to carry out the time consuming process on the roof inch by inch.

Restoring the Caryatid porch is expected to take one year, but the cleaning will continue as long as pollution persists.

"The conservation team will remain on the rock because the marble is alive. It will remain exposed to the atmosphere," said Papaconstantinou.

For years, archaeologists and scientists have debated how to protect the monuments from pollution, some even suggesting the temples be covered with domes. The creation of an Athens subway helped reduce pollution, but vehicles still cram the streets and the Greek capital remains blanketed in a thick smog.

Acid rain has eroded some fine details from the porous marble of the Acropolis sculptures, including the Caryatids, and have had to be moved to museums and replaced with replicas.

"We can't stop the pollution, but we can lessen the effects," said Ioannidou.