The stone arches looped solemnly over their shadows, some teetering above the grass, some sinking into it. I was standing in the Park of the Aqueducts in January, about 20 minutes by metro from central Rome. Here, the ruined arcades of six of the 11 aqueducts that once supplied the Eternal City with an astonishing volume of water — by some counts double the per capita water allotment of a typical 21st-century American city — have been preserved.

My aim was to trace the course of one of them: the Aqua Marcia, built between 144 and 140 B.C. by Julius Caesar’s ancestor Quintus Marcius Rex.

Hailed by Pliny the Elder as “the most famous of all waters in the world for coldness and wholesomeness,” the Marcia was also the longest of the capital’s ancient aqueducts, running some 56.8 miles from source to city. Only about 6.2 miles stood above ground.

I had always assumed that Rome’s aqueducts were a kind of aerial plumbing, their water channeled atop arches. But the Marcia, like all classical aqueducts, ran largely underground. The water moved by the force of gravity; arches and bridges, which were expensive and vulnerable to attack, were only used to span ravines, valleys and other dips in the terrain that would have interrupted the flow.

The arcade of the Marcia is now dry, but the same water still feeds mountain springs east of the capital through a modern network of tunnels and tubes. And it is still considered Rome’s best drinking water.

One of the series of arches that rose before me once carried this current to Rome. But which one?

Park of the Aqueducts

The roughly 600-acre Park of the Aqueducts has few signs, maps or directions. Romans come here to jog and walk their dogs. The few tourists wander through a bucolic landscape, green, tranquil, its imposing ruins seemingly untouched by modernity.

Michele Alfonsi, a lawyer who heads up Pons Iani, a volunteer group devoted to aqueducts, offered to guide me. “See that?” he asked, pointing to a stone passageway atop massive arches. “That’s the specus of the Aqua Marcia.”

Specus is the Latin term for a roofed channel built at a slight downward slope so that water would run through it without gushing or puddling. This one was nearly high enough to stand up inside.

We clambered up the keystone of the arch, now just a few feet above ground level. When it was completed during the heyday of the Republic, the Marcia was the first aqueduct to bring water to the Capitoline, Rome’s most sacred hill. A small fountain there has been chiseled with the words “Acqua Marcia,” but like modern Rome’s taps, it now spouts a mixture of water from five different founts.

To sample pure Marcia water, I’d need to travel to the source.

‘Where’s the water?’

Sextus Julius Frontinus, the first-century commissioner of the aqueducts, wrote that the fount of the Marcia is near the 36th milestone of the ancient Roman road Via Valeria (roughly 35 miles east of Rome). But I had been warned that the original trenches had been obliterated in 1870 when the Marcia’s long-defunct classical aqueduct was reincarnated as the Acqua Pia Antica Marcia.

“You’ll get close,” said Peter J. Aicher, author of “Guide to the Aqueducts of Ancient Rome,” “by searching for Centro Casetta Rossa Idrico on Google Maps.”

I found the “casetta,” a small red stucco house used by the modern aqueduct’s maintenance personnel, at the edge of a green field. Aside from the inscription “Acqua Pia Antica Marcia 1870” carved over the front door, and the shed-like structures built above springs alongside the road, there was no indication that Rome’s best drinking water originated here.

“Where’s the water?” I asked a maintenance man. He pointed down: The underground springs that Quintus Marcius Rex first channeled more than 2,000 years ago still bubble beneath this bucolic spot. The only way to plumb Marcia’s depths was to go spelunking.

Which is how, a few days later, I found myself clinging to an exposed tree root on the side of a ravine dropping to the Aniene River east of Rome. “Put your right foot there,” Alfonso Diaz Boj coaxed. “Two more steps and we’ll be at the Marcia’s specus.”

Diaz Boj, a guide with Sotterranei di Roma, which offers tours of Rome’s underground treasures, was leading a jaunt into the ancient, now-dry aqueduct channels buried near the town of Vicovaro, about 8 miles west of the Marcia’s source. Twelve of us met at the Convent of San Cosimato, whose property contains the ruins, to suit up in hard hats and headlamps.

Once we had negotiated the hand- and toeholds and were hunched into the shoulder-high specus, Diaz Boj pointed to a lozenge of light slanting down from a shaft: “Teams of workers excavated these shafts every 15 meters. When they reached the proper depth, two teams dug toward each other laterally until they joined up.”

Over the centuries, the Marcia’s water had deposited multicolored bubbles and stripes of calcium on the concrete that Romans used to seal the specus. Diaz Boj pointed to graffiti scratched into the concrete — mysterious crosses, doodles and the possibly faked signature of Thomas Ashby, the British archaeologist and author of the 1935 “Aqueducts of Ancient Rome.”

After lunch, I had a drink from a spigot in the garden. Only later did I learn that Vicovaro is inside the zone that receives the Marcia’s water unadulterated. It was delicious and refreshing, though I can’t say I detected much difference from the mixed water of central Rome.

Engineering feat

The Marcia entered Rome on arches at Porta Maggiore, chosen as the entry point for eight ancient aqueducts because of its high elevation on Esquiline Hill. At first glance this busy crossroads near the Termini rail station struck me as rough and forlorn, but I gave it a closer look. Aqueduct arches converge or radiate from every direction.

If you tune out the traffic, there is no better place to savor what one historian calls the Roman “knack for practical engineering on a monumental scale.”

It takes about half an hour on foot to trace the Marcia’s path through ancient Rome. From Porta Maggiore, the aqueduct tracked the Aurelian Wall as far as the elegant Augustan arch called Porta Tiburtina. From there, it veered off to follow today’s Via Marsala before emptying into a distribution basin now buried beneath the train station.

Fountain of the Naiads

After the Marcia was reborn as the Marcia Pia in 1870, the Fountain of the Naiads was conjured up to showcase its purity in the Piazza della Repubblica, a 10-minute walk from Termini station.

Some of Baroque Rome’s most cherished monuments are display fountains, or mostre, celebrating the newly restored aqueducts that once again brought spring water to Rome.

The Fountain of the Naiads is unique. Unlike the gravity-fed aqueducts of premodern times, the Marcia flowed under pressure created by mechanical pumps, which allowed the fountain’s jets to shoot nearly 7 feet high.

Katherine Rinne, the author of the forthcoming “Walking Rome’s Waters,” calls this “the Hugh Hefner fountain” because of its cavorting naked nymphs. It rises in the middle of a busy major intersection, but even if you get to it, the water isn’t drinkable.

Instead, if you are inspired to sip from the source, do as the Romans do and cup a hand under one of the nasoni (big noses) that spout from goose-necked spigots all over town. Two thousand years after Pliny lauded the Marcia as a gift from the gods, Rome’s aqueducts are still lavishing cold, clear spring water on the Eternal City.