In some of the most iconic cities in America, you can walk across entire neighborhoods that, from a geological standpoint, are not built on solid ground.

Once upon a time, places like the Back Bay of Boston, Battery Park City in Manhattan, Pioneer Square in Seattle and Chicago’s Grant Park were home to marshlands, tidal flats, riverbeds and other bodies of water. During their early years, these cities — all surrounded by or abutting water — were in dire need of additional land to build housing, parks, ports and so on. The solution was to create new land by simply "filling" in the shoreline with mud, sand, construction debris and other materials.

For example, the Shawmut Peninsula, the rather diminutive natural landmass that Boston is situated on, more than doubled in size thanks to 19th-century land reclamation projects in which massive amounts of gravel and dirt were dumped into wetlands. A more contemporary example is Battery Park City, a 92-acre residential enclave on the southern tip of Manhattan that was part of the Hudson River prior to the 1970s. Three million cubic yards of soil and rock later, a sizable man-made chunk of land was born. (The soil and rock came from the excavation of the World Trade Center site and other major New York construction projects.)

Mud from San Francisco Bay is dredged as part of a 1936 land reclamation project near Yerba Buena Island. Neighboring Treasure Island, which is man-made, is connected to Yerba Buena Island via a causeway. (Photo: Wikimedia Commons)

Another city with entire waterfront neighborhoods famously built atop fill is San Francisco. In fact, much of the Bay Area — not just the city proper — consists of land reclaimed from this densely populated West Coast region’s namesake bay. These man-made swaths of land are the most vulnerable to flooding unleashed by sea level rise, according to a new study published by scientists from the University of California, Berkeley and Arizona State University.

The fact that the Bay Area’s low-lying neighborhoods would be prone to increasingly severe coastal flooding isn’t surprising; it comes with the territory. But the potential for catastrophic flooding is much more dire than previously believed because parts of the Bay Area created with landfill are also sinking at an alarming rate. Double trouble.

"This is a big problem in the Bay Area," Roland Burgmann, the study’s co-author and a professor of Earth and Planetary Science at UC Berkeley, tells the San Jose Mercury News. "Quite a few of these areas are developed. In terms of long-term planning, it’s a huge problem."

That sinking feeling

North Beach is just one San Francisco neighborhood built atop fill. Mud, gravel, sand and earthquake rubble were used in the city's various land reclamation projects. (Photo: torbakhopper/flickr)

A natural but often disconcerting geological phenomenon known as land subsidence is the culprit behind the sinking sections of San Francisco. Subsidence is often associated with the after-effects of drought when underground aquifers have been pumped so dry that the land on top collapses. In this instance, it occurs on land that's still settling. As Burgmann tells local CBS affiliate KPIX 5: "These areas are more susceptible to sinking because they’re built on landfill that is compacting, water is being squeezed out of the rock, and that continues for many, many decades."

To be clear, most of the Bay Area — this includes "natural" sections not resulting from land reclamation — is sinking, albeit very slowly at a rate of roughly .06 inches per year. But as Burgmann and his colleagues have discovered using advanced satellite technology, the risk of severe coastal flooding has dramatically increased — even doubled — in areas where the land is sinking at a similar or faster rate to the rising sea. (The San Francisco Bay has risen a total of 8 inches since the 1850s.)

According to a UC Berkeley news release, previous flood risk studies that didn't take land subsidence into account found that between 20 and 160 square miles of the San Francisco Bay shoreline face severe inundation by the year 2100, a figure dependent on the rate at which sea levels rise. When considering the newly published data, the amount of vulnerable shoreline jumps from between 48 to 166 square miles.

"The ground goes down, sea level comes up, and flood waters go much farther inland than either change would produce by itself," explains Manoochehr Shirzaei, an assistant professor at the School of Earth and Space Exploration at Arizona State University who headed the study, alongside Burgmann.

Recent National Academy of Sciences and United Nations estimates shared by the Mercury News show the Pacific Ocean — and, in turn, San Francisco Bay — will rise one foot within the next 12 years and 2 feet within the next 32 years if the current amount of greenhouse gases entering the atmosphere hold steady.

Airports, islands and entire cities could be swallowed by the bay

Located directly on San Francisco Bay, San Francisco International Airport is at high risk of being inundated with future flooding unleashed by a warming planet. (Photo: Ashwin Kumar/flickr)

So, exactly how fast are the most vulnerable sections of the Bay Area sinking?

Located south of San Francisco in unincorporated San Mateo County on what was once marshland, San Francisco International Airport — the seventh busiest airport in the U.S. based on passenger count — is sinking so quickly that half of the facility’s runways could be under water by 2100 if sea level rise projections hold true.

Treasure Island, a neighborhood in San Francisco located on an artificial island. (Photo: Wikimedia Commons)

Floating smack dab between San Francisco and Oakland, Treasure Island is a 400-acre artificial landmass constructed from quarried rock and mud dredged from the depths of the bay. Built as a venue for 1939's Golden Gate International Exposition, the island was subsequently used as a Navy base and popular filming location for movies and TV. It’s also a unique — and, gasp, affordable — place to live with a close-knit population of roughly 2,000. Much to the chagrin of Treasure Island old-timers, that number is expected to explode due to a massive redevelopment scheme now underway. However, new arrivals to the cleaned-up and newly cool neighborhood be warned: the island, famous for its knockout views, is sinking at a rate of .5 and .75 inches annually.

According to Burgmann, the northwestern corner of Treasure Island in particular is experiencing "significant subsidence."

Another area of concern is Foster City, a prosperous planned city in the marshlands of San Mateo County that — you guessed it — was developed as a land reclamation project in the 1960s. Home to Visa and numerous tech firms, Foster City is sinking at a faster rate that its non-landfilled neighborhoods and will likely experience significant inundation by 2100.

Considered by many as part of Silicon Valley, Foster City was conceived in the 1960s as a land reclamation project. Today, it's a well-heeled city of roughly 34,000. (Photo: Mark Doliner/flickr)

Along with built-on-fill parts of San Francisco proper, other sinking Bay Area burgs mentioned in the study are shoreline-abutting parts of Hayworth and Union City, both in Alameda County, and the Alviso section of San Jose.

Scientists hope their findings, recently published in the journal Science Advances, will prompt the Federal Emergency Management Agency (FEMA) to update its flood risk maps to not only include projections based on rising seas, but also by sinking land. As the Mercury News points out, because subsidence is not accounted for in flood-prone areas, everyone — from insurance companies to homeowners to local and statewide emergency agencies — is underestimating the true risk at hand.

"Accurately measuring vertical land motion is an essential component for developing robust projections of flooding exposure for coastal communities worldwide," Patrick Barnard of the U.S. Geological Survey tells Berkeley News. "This work is an important step forward in providing coastal managers with increasingly more detailed information on the impacts of climate change, and therefore directly supports informed decision-making that can mitigate future impacts."

Inset aerial photo of Treasure Island: Wikimedia Commons