Walking past Newbury Street’s quaint shops or strolling Commonwealth Avenue’s boulevards, it may seem bizarre that Back Bay could be a recent creation. But until the mid-nineteenth century, the now opulent neighborhood was nothing more than a low tidal marsh.

The conception of Back Bay as a neighborhood first came to Boston as a geographic crisis. By the 1840’s, Irish immigrants made up a substantial portion of the downtown population, which was previously contained on the undersized Shawmut Peninsula, and the need for expansion became undeniable.

The problem was exacerbated by the flight of Brahmin elites. Unable to cope with the practical and cultural demands of immigration, many of Boston’s old families put pressure on the government to find a solution. Smaller areas around the peninsula had already been filled in, but city officials needed something grander, not only to accommodate the immigrants, but also to keep traditional elites downtown.

But ultimately, it took a more concrete crisis to fill Back Bay. In 1814, the Massachusetts Legislature had authorized the Boston and Roxbury Mill Corporation (BRMC) to build a dam across the Back Bay. On the promise of factories and jobs, the dam isolated the Bay from the Charles and further divided it into two basins, deriving energy from the ebb and flow of tides.

Though the dam was never popular, its propinquity to the city would be its undoing. At the time, local sewers were designed to dump their contents at the nearest shoreline, meaning that the isolated basins received a constant stream of garbage and excrement.

Over the following decades, the situation deteriorated to such an extent that an 1849 city committee described Back Bay as “nothing less than a great cesspool, into which is daily deposited all the filth of a large and constantly increasing population.”

To address the health crisis as well as the housing demand, the city established the Commissioners on Boston Harbor and the Back Bay, tasked to fill in the basin and cover the exposed sewage. Soon after, a series of agreements allocated work and land rights between the city, Commonwealth and BRMC.

Money, however, proved to be an enduring obstacle. Because the state legislature barred the project from receiving taxpayer money in 1856, the state was forced to sell land as they progressed. Instead of working west to east, which would have saved time and energy, the state began its work east to west, putting the newest parcels of land on display for wealthy Beacon Hill residents.

As the work dragged on, innovative technology soon came on full display. Shovels, handcarts, and gravity railroads of earlier decades were quickly replaced by the steam evacuator, a massive mechanical shovel that contractors George Goss and Norman C. Munson utilized. When a train arrived on site, which happened around-the-clock until 1863, two steam evacuators would dig out a chunk of land and dump it in the car. According to reports, a 35-car train could be filled in 10 minutes, replacing the work of nearly 200 men.

It took several decades, but the neighborhood slowly emerged.

By 1876, the state had finished the majority of its role and replaced $1.6 million in expenditures with a $5 million profit, nearly $90 million today, largely due to their focus on building upper middle class residences and preserving the value of Commonwealth Avenue.

Miscellaneous work continued for decades, but the old marsh quickly became a downtown staple. Thousands of residents now call Back Bay home and millions more walk its streets yearly, perhaps the reason many take the old marsh, larger than the Shawmut Peninsula itself, for granted.