The "Golden House" that Nero torched Rome to build. RELATED LINKS City Guide Rome

In a city where historical artifacts have quite literally been piling up in layers for more than 2,000 years, it's no surprise that some of the oldest and least accessible monuments are located beneath the streets of modern Rome.

The most striking example can be found beneath the ruins of Trajan's Baths, where archaeologists have excavated what remains of one of the most extravagant palaces ever built. The emperor Nero ordered construction of his Domus Aurea, or "Golden House," on land cleared by a devastating fire that flattened a large swath of Rome in 64 A.D.

The vast compound contained elaborately decorated pavilions and living quarters surrounded by gardens, vineyards and an artificial lake on the site where the Colosseum now stands. Much of the palace was destroyed after Nero's death, but some structures were simply covered over in the time of Trajan, and since 1999, the excavated ruins have been open to a limited number of visitors by advance reservation.

Guided tours lead through shadowy tunnels into a series of subterranean chambers decorated with exquisite frescos and mosaics. The Domus Aurea is open daily except Tuesdays from 9 a.m. to 7:45 p.m., with an adult entry fee of 5 Euros, plus a 1.50 Euro reservation fee. To reserve by phone, call +39-06-3996-7700 Monday through Saturday from 9 a.m. to 1:30 p.m. or 2:30 to 5 p.m. local time, or reserve online at www.pierreci.it.

A Rome-based travel company called Context|Rome offers an ongoing series of architectural and archaeological walking tours for small groups, including visits to the Domus Aurea and other lesser-known underground sites. The guides are members of Scala Reale, a nonprofit network of mostly American scholars and graduate students. For schedules and booking information, see www.contextrome.com.