Part of the ceiling of Nero’s 2,000-year-old Golden Palace collapsed in Rome Tuesday morning, leaving a huge hole in the ground.

The collapse occurred at the central vault in one of the galleries inside the fabled complex, in an area known as the “Fifteen Room.”

Experts believe water had seeped into the ceiling, causing it to literally crumble away.

“Around 60 square meters (645 square feet) have collapsed due to heavy rain. The damage does not involve the main part of the palace, but one of the galleries built by Trajan, the emperor who succeeded Nero, in 104 A.D.,” Antonello Vodret, technical director of the site, told reporters.

No injuries were reported, but the situation is cause for “extreme alarm,” according to Luciano Marchetti, the special commissioner for the site.

“More collapses are possible in a near future,” Marchetti said.

Known as Domus Aurea, the palace was built by Nero (37 A.D. – 68 A.D.) after the great fire in 64 A.D. as an act of megalomaniac self-aggrandizement.

Lying on a hill overlooking the Colosseum, the sprawling palace was called “Golden House” because of the amount of gold leaf which adorned much of it, testifying Nero’s lust for excess.

Indeed, the ceilings of its 150 rooms were once encrusted with pearls and covered in ivory, its mazes of passageways were lined with frescoes, while a 160-foot statue, the so-called “Colossus of Nero,” stood at the entrance.

The palace was completed in 68 A.D. — the same year the emperor committed suicide — but within a decade was stripped of its marble, jewels and ivory, filled in and built over.

On the site of Nero’s lakeside palace, Vespasian built the Colosseum, while Trajan’s Baths and Hadrian’s Temple of Venus and Rome were also built on top of the site.

Totally obliterated, the Domus Aurea was rediscovered in the 15th century after a local resident fell through the ground into the remains of the palace.

Last September archaeologists uncovered at the site what they believe to be the remains of Nero’s “coenatio rotunda,” the famous rotating dining room which, according to the ancient historian Suetonius, rotated day and night to imitate Earth’s movement.

But the monument has been always plagued by structural problems. It was reopened in 1999 after an 18-year restoration project following similar problems over rainwater seeping into the brickwork.

It was briefly closed to the public again in 2001, after part of a ceiling collapsed. In 2005, the monument closed again after heavy rain threatened to cause other collapses. Briefly reopened the following year, it was closed again in 2008.

Following a 3.2 million-euro ($4.3 million) restoration project, the Domus Aurea was scheduled to re-open completely in 2011.

Experts now estimate that it will cost some 10 million euros ($13.4 million) to repair the water infiltration problem and restore the monument.

Picture: A detail of what is believed to be the famous rotating dining room of the Golden Palace. Courtesy: Soprintendenza Speciale per i Beni Archeologici di Roma.