Why do so many Romans have nice teeth? Why do they laugh at a well-timed quip in such a carefree way? The answer lies in the hills and mountains around them, guardians of the springs that provide them with the water they drink, water of a quality they are all proud of. I dont know why we drink so much mineral water when Romes water is so good, you sometimes hear them remark. How do their teeth come into it though? Because, as every dentist in Rome will confirm, Romes water is rich in minerals, above all calcium, the ingredient that keeps the smiles of dazzling health going for a lifetime. The water, they might volunteer, packs 97 mg of calcium per litre, against 19 mg/l of magnesium and 15 mg/l of sulphates.

Romes water distribution is run by ACEA (Azienda Comunale Elettricit Acqua), the same company that supplies the city with electricity. With a staff of 1,000, it serves not only Rome but 111 nearby towns. Others further afield in Lazio such as Latina and Viterbo come under a small archipelago of rival water boards.

But perhaps many Romans have been led to believe, as was this writer until a few days ago, that their water was a home-grown product, issuing forth from pristine springs in the nearby volcanic Alban Hills to the south of the city, a crown of heights around massive Lake Albano.

Wrong! answered Giorgio Signore at the headquarters of ACEA next to the Piramide in the Ostiense area of the city. The belief is badly out of date.

Since 1949, Signore disclosed, some 70 per cent of Romes water supply has been tumbling in from the most copious spring in the whole of the Apennine mountain chain, known as La Sorgente del Peschiera, some 90 km away, to the north of Rieti northeast of Rome. At a modest altitude of 410 m, the secret of Peschiera is that it is in a limestone, or Karst, area of porous rock which is riddled with caverns and tunnels hiding underground watersheds, so that the supply is far greater than it appears from its lake on the surface.

As the water percolates through the spongy rock, it sheds its waste elements and is progressively purified until it is among the purest in Italy, explained Signore. Even so, it still goes through filters at a big plant at Ottavia in the northwestern suburbs of Rome.

As well as Peschiera, the citys water comes from four other big springs: Capore (Apennines), Acqua Marcia (Castelli Romani), Aquoria (near Tivoli), and Salone (near Naples). At the water board they boast that the huge volume of water in the citys system at any given second would be enough to fill a lake of 50 sqkm and 10 m deep.

So why is such exemplary water being squandered on the 1,500 striking ornamental fountains in the city?

The reassuring answer is that it is not. Take the splendid Trevi fountain, said Signore. It is fed by one of the three old Roman aqueducts still functioning, the Acqua Vergine originating from the first century BC, but it arrives at the fountain raw, untreated, and so, for instance, does the water for the spectacular semi-circular Fontana dellAcqua Paola, commissioned by Pope Paul V and built on the Janiculum in 1610-12.

But the tourists drinking from the Fontana della Barcaccia at the foot of the Spanish Steps? No problem, apparently, since it is safe drinking water, like that at all the wrought-iron fountains, or nasoni as they are called, scattered around the city. Theirs is exactly the same uncontaminated water that spurts out of residents taps, purified and constantly analysed.

But why do so many of the fountains seem to be pouring out water day and night?

Its not a waste. They serve a specific purpose, was Signores surprising reply. It helps to keep the system on the move, mainly to cut down on calcium deposits, which, given the speed the water is shifted, are almost negligible.

No wastage at all then?

Well, we have an extensive network of aqueducts and pipes. Many of them are old and in need of permanent maintenance. Waste is inevitable but we reckon we dont lose much more than ten per cent.

Not that ACEA escapes catcalls from the public. In October, the inhabitants of Frosinone, south of Rome, were up in arms, alarmed about how their water was tasting worse and worse. ACEA investigated and found that because of an apparent fault at the local purifier, they were drinking contaminated water. In November, they were still working on the tricky problem.

The fear of possible terrorist attacks on the water supply seems to have been behind a drastic revision ACEA has made of its system. A new pipeline into the capital has been laid from Lake Bracciano on its northern approaches, with pipes 55 m deep in reinforced concrete wrapped in metal. Its mainly intended for use in an emergency and, like every other plant since 9/11, its very heavily guarded against sabotage. They wont even let me in! exclaimed Signore.