KEY POINTS:
Did you know that the vast network of catacombs dug under ancient Rome was built as a giant underground cemetery but now contain hardly any remains? By contrast, the church of Santa Maria della Concezione has several underground rooms built as chapels which have ended up full of bones.
As that indicates, Rome is a city full of fascinating contrasts, and many of the most intriguing are underground.
That's hardly surprising for a place which has been occupied for around 3000 years, sacked and rebuilt countless times, with new buildings constantly being erected on top of the old.
Probably the best place to get a feel for that is the Church of San Clemente, where you can visit a beautiful 12th century basilica, built over the top of a 4th century church, siting on a Roman house where early Christians used to meet, alongside a Mithraic Temple, under which is a still-running aqueduct - and who knows what else - dating back at least 2000 years.
I visited the church in the course of a tour entitled Crypts and Catacombs, which promised to unveil the secret underside of Rome and certainly confirmed that there's a lot more to the old city than meets the eye.
For instance, San Clemente is, at first glance, merely another of the hundreds of interesting old churches, but it sits on a network of excavations which extend down 17m and 2000 years.
This extraordinary layer of history was discovered 150 years ago by the church's Dominican superior, Fr Joseph Mullooly, apparently because he was puzzled by the constant sound of running water.
"Listen," said our guide, "and maybe you can hear it." We listened ... and we could.
In search of the water, Fr Mullooly dug - and instead found a 4th-century basilica complete with medieval frescoes showing the life of St Clement, which had been ransacked by Norman invaders in the 10th century. Nearby were the remains of St Clement himself, thought to have been the third Bishop of Rome, after whom the church was named.
Still pursuing the water, Fr Mullooly dug further and uncovered a 1st century Roman house, once owned by a senator who was an early convert to Christianity.
On the same level was a Mithraic temple, whose altar still bore the carved symbol of the god Mithras slaying a bull, where other Romans worshipped a religion which at the time rivalled Christianity.
Descending further into the depths, down narrow, ill-lit stairs carved into the rock, we came to a fourth layer where, sure enough, there was an ancient aqueduct still running with water.
"We don't know the source," said our guide, "but it takes water to the colosseum."
What else might they find if they keep digging?
"The next layer down is probably from the republican period. If we go deeper than that ... who knows?"
The catacombs do go even deeper, having been excavated down to four storeys and 20m over a period of around 200 years.
Mr Michael, the German theology student who showed us through the catacombs under the monastery of Santa Domitilla, was quick to dispel any misconceptions before we went underground.
First, he said firmly that whatever we may have been told, the early Christians did not live here.
"This was a place of burial. The stories that Christians hid in the catacombs were started by early English tourists doing the Grand Tour who found knives and forks and tables and thought that meant people lived here. But the knives and forks were for them for their luncheon."
Second, he said we would not be seeing any bones.
"There were perhaps 100,000 people buried here, but the graves are now 95 per cent empty and the few that are still occupied are sealed off with grave plates.
"Some of the graves were broken into by Ostrogoths and Vandals looking for treasure. But most were robbed by grave robbers to sell as martyrs' bones."
Caveats duly issued, he led us underground into a huge church, built around the 4th century, lined with carvings, frescoes and symbols telling Biblical stories or the tales of early martyrs, and still used for worship.
Indeed, as we walked through the church, the sound of chanting echoed up the passageways, apparently coming from a smaller chapel further down where a special mass was being performed for a group of visitors.
A nearby gallery decorated with grapes and birds was originally a pagan burial area which was taken over by the Christians, who added drawings of Moses and Daniel.
A large chamber with stone tables on three sides, and the remains of paintings on the wall, once hosted funeral feasts.
From there Mr Michael led us down, ever deeper, through a series of narrow passages, most lined with tiny niches which once held bodies - "people were much smaller in those days" - a few still covered with stone plates carved with representations of the life of the person inside and even the name of the platemaker.
One poignant little plate carrying a carving of a dog and a bird was apparently the last resting place of a 4-year-old girl. Another, now lost, once covered the grave of Diogenes the Gravedigger, showing him in his work clothes with a pick, an axe and a shovel.
As we descended further, there were turnings and openings everywhere, prompting Mr Michael to warn: "Stay close together because you could easily get lost." He needn't have worried.
There mightn't have been skeletons lying about, but the place still had a spooky air and you wouldn't want to be claustrophobic.
The contrast could not be greater with the crypts under the church of Santa Maria della Concezione. They are packed with the bones of 40,000 Capuchin monks, but somehow, rather than being scary, the atmosphere is ... light-hearted.
This is because the bones have been made into an amazing series of designs - flowers, a clock, lampshades, pictures on religious themes - which are a source of amazement rather than horror.
On the wall of one crypt stands a skeleton holding a scythe in one hand and scales in the other (both made of bones) surrounded by an oval frame made from more bones.
In another crypt, three monks in their habits stand before a wall of skulls, topped by a winged hour-glass (all bones again), while along the walls two more monks lie in beds of bone.
Yet another has the Franciscan coat of arms surrounded by a display of crosses, rosettes and flowers made from shoulderblades and vertebrae.
It's generally agreed that the bones were mostly brought here in 1631, when the Capuchin friars moved to this friary from one near the Trevi fountain, though some were also rescued from the ravages of the French Revolution.
But how did they end up being transformed into this remarkable work of art? "I've heard five explanations," said Katrina, our guide.
"But the one I prefer is that a man hiding from the authorities was given refuge down here for several years and he did this to pass the time and as a gesture of thanks to the monks."
As gestures of thanks go, this one's a bit unusual.
But then there are a lot of unusual things beneath the surface of modern Rome.
Jim Eagles went to Rome with help from Air New Zealand. Air New Zealand (www.airnz.co.nz) flies daily via Hong Kong or Los Angeles to London, with connections to Milan and Rome with partner airline Alitalia.
