It was one of the biggest civic projects of its day, destined to transform Boston's (search) burgeoning urban landscape and fulfill the dreams of city dwellers desperate to embrace a modern era.

But unlike that other big Boston public works project 150 years later, the filling of Boston's Back Bay (search) — which began in earnest in the summer of 1855 — not only wowed the public, it also turned a tidy profit as reclaimed land was sold.

It's hard to imagine that the massive $14.6 billion Big Dig (search) could have been built on the Back Bay's pay-as-you-go model, but the two projects share a grand ambition: Each was envisioned as the best possible fix for a dire civic need.

In the mid-19th century, Boston's problem was twofold: the flight of the upper classes from a city crowded by waves of immigrants and the noxious buildup of waste in muddy tidal flats abutting the Charles River.

In the 1980s, the problem was a clogged and outdated highway system that city leaders feared could choke off economic growth.

The contrasts are sharper.

The Back Bay, with its Parisian boulevards, stately Victorian townhouses and grid layout, is almost universally seen as an architectural jewel. It's one of the most desirable, and pricey, places to live in Boston.

The Big Dig — with its graceful new bridge and its tunnels running along Interstate 93 below downtown and linking the Massachusetts Turnpike and Logan Airport — draws attention for its engineering. But its much briefer legacy also includes soaring costs, political infighting and construction blunders.

By mid-century, it was clear something had to be done about the Back Bay. A mill built along the edge of the marshland years earlier had turned the tidal mud flats into an open sewer.

A city report from 1849 described the area that would become one of the city's ritziest as: "a great cesspool, into which is daily deposited all the filth of a large and constantly increasing population ... while the surface of the water beyond is seen bubbling like a cauldron with the noxious gases that are exploding from the corrupting mass below."

The solution — unthinkable today — was to fill it in.

Adding to the urgency were the boatloads of poor immigrants fleeing famine-ravaged Ireland. They were filling up the city, driving out older Yankee families used to more genteel living.

In an effort to stem the flight, the state — which controlled about a quarter of the mud flats — began shipping in gravel from suburban Needham to fill them in. During the first five years, trains made the nine-mile trip day and night, gradually covering the noxious wasteland with clean gravel.

The first block reclaimed was facing the Public Garden across newly formed Arlington Street. The state sold the lots at auction, typically to the city's Brahmin elite, and used the profits to continue funding the fill project.

The process would repeat itself for decades. Anyone watching from the Statehouse atop Beacon Hill could see the four- and five-story townhouses gradually sprouting westward toward what is now Kenmore Square.

Architectural students walking the length of the Back Bay today can see written in stone the slow progression of building styles.

To make the area even more desirable to the upper classes, planners created the grassy Commonwealth Avenue Mall, modeled on the French boulevards of the period. They also required that some of the land be dedicated to cultural institutions. The site now occupied by the Copley Plaza Hotel once housed the Museum of Fine Arts.

The experiment created one of the nation's handsomest neighborhoods and reaped a 400 percent profit.

"It achieved its purpose of trying to keep the Brahmins in the city," said Nancy Seasholes, author of "Gaining Ground, a History of Landmaking in Boston."

Jeff Stein, director of the school of architecture at the Boston Architectural Center, said the filling of the Back Bay was more important to the image of Boston than the Big Dig.

"It was a much larger area ... and it allowed thousands more people to actually live in Boston," he said. "The Big Dig is a way for people to get through the city without ever experiencing the city."

Even the Big Dig's demolition of the hulking, elevated Central Artery, which walled off the harbor and some historic neighborhoods from the city center, and the promise of new parkland where it stood isn't enough to raise its historic standing in the eyes of some critics.

"The Back Bay showed a great city surging toward an immense bursting of achievement. The Big Dig is nothing remotely like that," said Boston historian and author Douglass Shand-Tucci. "With the Commonwealth Mall, the point was to create the greatest street in America. With the Big Dig the idea was to create a good tunnel system and everything else was an afterthought."

Not so, according to defenders of the Big Dig. They point to the elegant spans of the Zakim Bunker Hill Bridge, which drew hundreds of thousands of pedestrians for a walkover before it opened to traffic, and the 30 acres of prime downtown land now being developed as a park.

"You are hard-pressed to see any picture of the city without the Zakim Bridge. It's a signature piece, it's become an image of the city of Boston," said Mariellen Burns, a spokeswoman for the Massachusetts Turnpike Authority.

"People need to stop thinking about this as the Big Dig. People have to start thinking about this as a new Boston because that's what being created here."