For a little early Christian history, we're heading outside the city for a look at the catacombs.

Rome's ancient wall stretches eleven miles. It protected the city until Italy was united in 1870. From gates like this, grand roads fanned out to connect the city with its empire.

The Appian Way — Rome's gateway to the East — is fun to explore on a rented bike. It was the grandest and fastest road yet… the wonder of its day. Very straight — as Roman engineers were fond of designing — it stretched 400 miles to Naples and then on to Brindisi, from where Roman ships sailed to Greece and Egypt. These are the original stones.

Tombs of ancient big shots lined the Appian Way like billboards. While pagans didn't enjoy the promise of salvation, those who could afford it purchased a kind of immortality by building themselves big and glitzy memorials. These line the main roads out of town.

Judging by their elegant togas, these brothers were from a fine family. This is the mausoleum of Cecilia Metella, whose father-in-law was extremely wealthy. While it dates from the first century BC, we still remember her to this day… so apparently the investment paid off.

But of course, early Christians didn't have that kind of money. So they buried their dead in mass underground necropoli — or catacombs — dug under the property of the few fellow Christians who owned land.

These catacombs are scattered all around the city, just outside the walls. And several are open to the public.

The tomb-lined tunnels of the catacombs stretch for miles and are many layers deep. Many of the first Christians buried here were later recognized as martyrs and saints. Others carved out niches nearby to bury their loved ones close to these early Christian heroes.

By the Middle Ages, these catacombs were abandoned and forgotten. Centuries later they were rediscovered. Romantic age tourists on the grand tour visited by candlelight and legends grew about Christians hiding out to escape persecution. But the catacombs were not hideouts. They were simply low budget underground cemeteries.

Further along the Appian Way is Rome's Aqueduct Park and a chance to see how the ancient city got its water. With its million people, Rome needed lots of water. These ingenious aqueducts carried a steady stream from distant mountains into the city. And they still seem to gallop, as they did 2000 years ago, into Rome.

These aqueducts were the Achilles heel of Rome. All you had to do to bring down the city was to knock out one of these arches. In fact, in the 6th century, the Barbarians did just that. Without water Rome basically shriveled up.

Today, the park is a favorite with locals for walking the dog… or burning off some of that pasta.