BBC News, Rome
They were digging the foundations of a new multi-storey car park under the Vatican.
Dionysus' mosaic beneath the construction site
But what the bulldozers uncovered was an ancient world of the dead - a Roman necropolis, or burial site, dating back to the time of Christ.
Since then the excavations have brought to light more than 200 tombs, arranged on multiple levels and in remarkable condition.
In addition to funerary inscriptions they have uncovered a wide assortment of statues, vases, terracotta urns, coins and skeletons.
The burial site paints a complex picture of life and death in ancient Rome and for the first time gives archaeologists a valuable insight into the life of lower- and middle-class Romans.
Some of them were simple artisans, buried with clues to their trade.
In the tomb of a set designer for Pompey's theatre, there are the symbols of a compass and a T-square.
Urns, vases and statues were discovered during the excavation
There are the tombs of a letter carrier, a circus horse trainer and a slave who was freed and later rose to a respected position in the household staff of the Emperor Nero.
But they have also uncovered skeletons of paupers, possibly slaves, who were buried without names and in simple wooden caskets.
"We found a little Pompeii of funeral life, " said Giandomenico Spinola, head of the Vatican Museums' classical antiquities department.
"We have had the mausoleums of Hadrian and Augustus," Mr Spinola said. "But in Rome we are short of these middle- and lower-class burial places."
In some parts of the necropolis lie the tombs of much wealthier Romans. Some of them are complete with ornate funeral altars.
The inscriptions help to fill out family trees and they provide an important insight into daily life.
The necropolis dates back to the time of Christ
There is the sarcophagus of a male member of ancient Rome's class of knights, who died as a teenager and was remembered in death with a sculptured figure with hands outstretched as if in prayer.
The kind of figure known as an orante was widely taken as an early symbol of Christianity.
Before archaeologists could begin the excavation, they had to clear tons of dirt and rock.
In the second century there had been a landslip on the hill which helped preserve some of the tombs.
Black-and-white mosaic flooring was unearthed depicting Dionysus, an ancient god of fertility and wine, along with a grape harvest scene.
It has all been carefully restored in the Vatican Museums' laboratory and placed back in its original location.
From specially constructed walkways, visitors can look down on the skeletons, including that of an infant buried by loved ones who left a hen's egg beside the body.
The egg, whose smashed shell was reconstructed, was either a play thing or perhaps was left by the family as a symbol of rebirth.
Throughout the necropolis there are a number of terracotta pipes that emerge from the tombs.
Paupers and slaves were buried in unnamed graves
In ancient times families would sit by the grave and picnic, occasionally pouring wine, milk or honey down the pipe to feed the dead.
Originally the necropolis ran along the edges of an ancient Roman road, the Via Triumphalis (Triumphal Way).
Now we know that the area uncovered is just a small section of a much bigger necropolis that would have covered a large part of the hill.
But many of the secrets will remain buried.
Archaeological digs like this are expensive - the work carried out so far has cost the Vatican around 400,000 euros (£268,851) - and the current site is now surrounded by the imposing stone pillars of the new multi-storey car park.
The advance of the modern world has, for the time being, put paid to any further excavations.