Weeds with stone-splitting roots. Relentless traffic belching pollution. Tourists trampling across the once palatial residences of emperors. Earthquakes and terrorism waiting to happen.

From the imposing stone bulk of the Colosseum to the romantic ruins of imperial luxury atop the Palatine Hill, the Eternal City's monuments, once pillaged by foreign conquerors, today face an array of perils old and new.

Rome's fragile ruins have the urgent attention of teams of monument "doctors," armed with such high-tech instruments as micro-cameras probing for weak spots.

So far, the Colosseum has made it through two millennia, its imposing stone bulk still standing after quakes, lightning strikes, pillaging, traffic tearing round it and subway cars vibrating below. And now, following the terrorist bombings in London and Madrid, the great stadium where gladiators once thrilled the masses is equipped with metal detectors.

"The Colosseum is always worrisome because of the threat of an earthquake," said Giorgio Croci, an engineer who has been studying it for years.

Topping the experts' list of potential perils these days is the Palatine Hill.

"The Palatine is an area extremely dense in monuments in a more precarious state," said Croci in an interview in his studio on the Aventine, another of ancient Rome's seven hills.

"Frightening" and "terrifying" are the words used by Giovanna Tedone, an architect for the Palatine from the state's archaeology office, as she points out fissures and piles of crumbled brickwork during a walk around the towering ruins.

Roots of wildflowers and weeds bore through brick, and rainwater seeps through stone, forcing authorities to close most of the Palatine's 67 acres to tourists climbing up from the Roman Forum.

Green netting encloses a section of crumbled wall, built by the aristocratic Farnese family in the 16th century, which collapsed along the edges of the Domus Tiberiana in November. The wall gave way at night, when the Palatine was closed, and no one was hurt.

"We had the gods on our side," said Irene Iacopi, the archaeologist in charge of the Palatine and the Roman Forum.

Four architects and an engineer have spent months poking the Palatine's insides and monitoring cracks, using endoscopes similar to those that detect disease in human innards. The technology "helps us to do what we couldn't do before," says Angelo Bottini, the state's top official for archaeology in Rome. "All you need is to make a little hole in a wall, put in a probe and you get an image of the inside."

Excavations on the Palatine in recent decades have turned up wonders such as the Emperor Augustus' house, including two rooms with stunning frescoes of masked figures and pine branches which archaeologists hope to open to tourists once the Palatine is safer.

Much of the Palatine is still unknown, especially its underground passageways.

"We don't know where the tunnels end," said Croci. "In some tunnels there are frescoes covered with dirt. There is still a world to explore."

The national budget, sagging under the cost of generous pensions and health care, can't keep up with the pace of archaeology in a city where "every day there's a discovery," Culture Francesco Rutelli, a former Rome mayor, said at the unveiling of a recently excavated 7th century B.C. frescoed Etruscan tomb of a warrior prince.

Archaeology authorities get to keep 80 percent of ticket sales at Roman sites, but the income doesn't cover the costs of preservation, said Bottini, the archeology official.

When results of the $1.25 million Palatine mapping project are turned in this month, the monument doctors will start checkups on other sites: the ancient forums, Trajan's Markets, Nero's Golden Palace and the Colosseum.

The Domus Aurea, as Nero's palace is known, reopened six years ago after a $3 million restoration, only to be closed again in December when heavy rains put it at risk of collapse.

While Croci says the Colosseum "has an incredible, extraordinary resistance," Rome has an earthquake every few centuries, the last at the start of the 18th century.

The engineer said relatively cheap measures could improve its safety, such as cables sunk vertically down the stone as anchors.

"From an engineering standpoint," he said, "Rome's monuments can go on for 10,000 more years."