Excavations near the antique city of Vindunum (now Le Mans) have revealed a vast religious site dating from the first to the third centuries AD with remarkably well-preserved offerings.

Sometimes archaeology requires imagination. And you need it to conjure up the vast complex of temples that stood nearly 2,000 years ago on this flat two-hectare strip of land, in what is now Neuville-sur-Sarthe, 4km to the north of Le Mans.

"I have been an archaeologist for 30 years, and I've been lucky enough to work on some wonderful digs. But this is an exceptional discovery, the sort that all archaeologists dream of making once in their lives," said Gérard Guillier, who heads the team from the National Institute of Preventive Archaeological Research (Inrap) that has been poring over this piece of land since June. The team has no time to lose because in the autumn this former Gallo-Roman sanctuary will be transformed into an "urban development zone".

After an aerial assessment that revealed the shape of the ancient buildings in the wheat fields, followed by the some underground probing, mechanical diggers were sent in to clear the surface of the site. Unfortunately the blocks of limestone and sandstone from the antique buildings had disappeared, salvaged over the centuries for other building work in the area. Only a few stones bear witness to the original temple structures. Young archaeologists uncover them delicately one at a time, using trowels, scrapers and brushes. Every stone is numbered, drawn and its location marked on a map.

"Given the size of the site, hundreds of pilgrims, possibly thousands, would have come here to honour the gods," said Guillier. "They probably held other mass events here too."

The lines drawn on the ground by the archaeologists make the site resemble a vast treasure hunt. The red ones indicate the streets, paths and galleries that once connected the buildings, while blue circles mark the holes that held the pillars supporting the colonnade, which led the visitors to the temples.

At the entrance to the site, there once stood a large E-shaped building, probably for welcoming the pilgrims, selling religious objects and housing the temple guardians. One wide path littered with iron slag (Vindunum was a major metalworking centre), leads a few hundred metres south to the foundations of a circular fanum (temple) about 12 metres in diameter. That round shape was rare in Gallo-Roman times and there are only a few such examples in France.

In fact, three temples were erected successively during the second and third centuries. Possibly they had to be rebuilt because of the instability of the ground. A pergola and a flight of steps would have led to the temple, which had stone walls around seven metres high covered by a tiled roof. Inside, the cella (central room) housed the statue of the god.

Another fanum stood at the west, the oldest in the sanctuary, dating to the first century. It was square, 15 metres wide and apparently in the Celtic temple tradition. This one was originally built in wood and stone added later, together with a cella surrounded by a gallery for circumambulation and a wall separating the sacred space from the profane. Fragments of coloured plaster show that the walls were once panted. The temple was surrounded by octagonal or square-shaped secondary "chapels".

It is here that the archaeologist uncovered a marvellous selection of objects placed as offerings. They include Gallic, Celtic and Roman silver coins, bronze and silver-plated bronze fibulae (broaches), some jewellery including a gold ring with a green quartz representing a deity, as well as bronze keys, pottery and knives. They also found a dagger, sledgehammers and hammers, possibly offerings from soldiers and ironmongers, who held high-risk occupations requiring more divine protection than others.

But what gods were worshipped there? No statues or inscriptions have been found as clues, and the Gallic pantheon was as plentiful as the Roman one.

Another large sanctuary once stood in Allonnes, to the south of Le Mans, dedicated to the Gallo- Roman god Mars Mullo. Would there have been two major sanctuaries in one city? According to Guillier, "Situated as they were on hillocks on either side of Vindunum, they probably had a protective role for the town."

The archaeologists have another enigma to solve. They have uncovered several graves near the circular fanum, with funerary objects such as a glass bottle and a box for seals. Until now archaeologists have never found temples and graves in such close proximity, since Romans observed strict separation between what they perceived as the "pure" and the "impure". It will take years to reconstruct the history of the sanctuary and its pilgrims. And a great deal of imagination.

This article originally appeared in Le Monde