width=372Thessaloníki, in northern Greece, is shaped by what lies beneath its modern façade of high-rises and traffic-choked streets. The contours of geography have defined the physical plan of the city: From downtown, which faces the steely waters of the Gulf of Thessaloníki, the urban sprawl pitches sharply upwards, clinging to a steep hillside. Amid the contemporary clutter, isolated bits and pieces of the past remain. For observant visitors, the layers of history are there to see.

Sometimes they are hidden in plain view. In the center of the city there is a huge, cylindrical, brick building. Beside it is what looks like a chimney. A 19th-century factory, perhaps? How uninteresting. The temptation is to walk by without a second glance, but if you did you would be missing a World Heritage site.

This apparently industrial building is in fact the Rotunda, a mausoleum built in A.D. 306 for the Roman emperor Galerius, though it was never used for its original purpose. For 1,200 years it was a Christian church, and then under the five-century rule of the Ottomans it was a mosque (the chimney is, in fact, a minaret). When the Greeks regained control of the city in 1912, the Rotunda became a church again - one of the oldest in the world.

A short walk away, the Arch of Galerius, a triumphal Roman monument carved with battle scenes, frames a major thoroughfare. Go along that street, turn off into the park, and you reach what appears to be a construction site. In fact, it is the extensive uncovered remnants of the old Roman market.

And so it goes on: here and there, evidence of the Macedonians, the Romans, the Byzantines and the Ottomans. Thousands of artifacts from this long and varied history have been collected together in the excellent Archaeological Museum.

The 20th century was not kind to Thessaloníki. After the Ottoman defeat of 1912, the long-established Muslim population was evicted from the city. In 1917, French soldiers accidentally burned down the Old Town, destroying many priceless treasures. And in 1941, the Nazis wiped out the sizeable Jewish population, sending around 45,000 to Auschwitz.

Memories of that lost population are preserved in the intensely moving Jewish Museum of Thessaloníki. Rescued headstones from the Jewish cemetery poignantly line the entrance hall. Upstairs, letters, personal effects and concentration camp uniforms eloquently record the tragedy.

In Thessaloníki, what is not here is as important as what is. Greece's second-largest city is haunted by the ghosts of its past.