Why are there so many Greeks in Tarpon Springs, Fla.? Because in the early 1900s, Greek sponge divers came from the Dodecanese islands and revolutionized the sponge industry on Florida’s gulf coast.

What explains the pockets of Portuguese and Cape Verdeans in New Bedford, Mass.? In the 1800s, winds pushed whaling boats east to the Azores and Cape Verde, where experienced whalers joined the crews.

There’s the Basque population in Boise, Idaho, whose ancestors traded a mountainous region between France and Spain for the American West in the hopes of finding gold but later turned to sheep herding. There are the families of Yemeni immigrants hired by Ford Motor Company to build cars in Detroit, and the Vietnamese refugees who were resettled near New Orleans and Houston, where they could carry on shrimping.

These stories are everywhere on this map of American ancestry, which shows how people described their backgrounds to the Census Bureau. There are nearly 200 unique identities represented; blend them — as 340 million Americans do — and we arrive at a jumbled, overlapping, story-filled infinity.

Much of what we see is a history of immigration. Over 250 years, the country has absorbed more than 100 million people. We can trace the pressures that pushed and pulled them here — and the policies that welcomed certain groups while keeping others out — through the patterns in where their descendants live today.

Now, a larger share of the country was born abroad than ever before, and the Trump administration’s sweeping immigration bans echo exclusionist policies enacted in response to similar demographic conditions a century ago.

Those policies defined Americans for generations. Recent efforts to limit immigration will likewise affect how future Americans understand their heritage and themselves.

How we got here

In the late 1700s, the area that would become the present-day United States was already diverse. At least 1.5 million Native people, and possibly many more, were living across the territory. They were joined by about three million Europeans and enslaved Africans living in both the English colonies and the French and Spanish territories.

From colonial times, immigration was an important contributor to population growth. It accelerated as the new country’s territory expanded west and immigrants arrived to settle it. From 1820 to 1860, more than five million people came, through a mostly open door.

With the advent of the steamship, the cost of passage plummeted, and companies offered special immigrant fares that were often coupled with rail tickets to the interior of the country. Once a community of immigrants was established somewhere, it tended to grow.

After 1840, immigration from Western Europe began to rise quickly as political instability in Germany and the famine in Ireland drove people to leave. Asian immigrants, drawn by the discovery of gold in California in 1848, were recruited to work on farms and railroads.

Later in the 19th century, pogroms across Eastern Europe and the aftermath of Italian reunification drove a surge of migration to the United States. From 1880 to 1920, 24 million immigrants arrived. They went almost everywhere except the South, where the land-owning elite already had cheap labor from the formerly enslaved and poor tenant farmers.

Cities swelled. In 1910, according to the Ellis Island National Museum of Immigration, three-quarters of the residents of Boston, Chicago, Cleveland, Detroit and New York City were immigrants or children of immigrants.

Many rural areas in the Midwest had a similar share of immigrants in 1910, but newcomers to the cities tended to be from novel sources like Russia or Italy. That meant there were more languages, more cuisines and more workers. It also meant there were more crowds, more slums and more people behaving in unfamiliar ways — fodder to drive views that the new immigrants were unassimilable and that policies were needed to keep them out.

The first federal law to severely limit immigration had come much earlier, in 1882, when practically all Chinese people were barred from entering the country. More restrictions followed, and eventually animosity toward new immigrants led to the passage of laws in the 1920s creating a quota system tied to nationality.

Western Europeans were given generous quotas and Southern and Eastern Europeans much smaller ones. For the rest of the Eastern Hemisphere, the quotas were set to almost nothing. Ships raced through the night to reach New York Harbor, all trying to be first to dock at Ellis Island.

There weren’t quotas for countries in the Americas and the Caribbean, but there were other restrictions. Mexicans faced mass deportation campaigns in the 1930s and 1950s, even as millions were recruited as temporary workers to fill agricultural jobs across the Southwest.

Over the next 40 years, these rules drove the foreign-born population in the United States to its lowest levels. Children of immigrants replaced immigrants, blending into American society while retaining their own cultural traditions.

Then, alongside the civil rights movement of the 1960s, activists and lawmakers who saw the national quota system as racist pushed to replace it with one based on employment and family ties.

Another decades-long wave of immigration followed, this time from different parts of the world.

The new rules allowed people to sponsor their family members and relatives, and they gave preference to workers with advanced degrees and specialized skills. The family visas, in particular, led to an unforeseen boom in immigration.

An expanded refugee program also brought more immigrants, many from Southeast Asia who were displaced by Cold War conflicts.

For the first time, immigrants from the Western Hemisphere faced limits on their numbers. Similar to the European workers who arrived earlier in the century, many chose to settle in the United States permanently instead of risking returning to their home countries between periods of working in the United States. Millions who couldn’t get visas turned to entering illegally.

The most recent immigration wave, during the Biden administration, was different still: The number of visas for immigrants remained steady, while migrants from Central America arrived at the southwest border in large numbers to seek asylum. Desperate conditions in Cuba, Haiti and Venezuela, as well as wars in Afghanistan, Ukraine and elsewhere led hundreds of thousands of people to flee to the United States — many of them drawn to established communities of immigrants from their countries.

Where we are today

The lines of American ancestry today are not neatly drawn, and groups overlap and spill into one another. Some people don’t answer the census questions about their origins at all. For others, it’s complicated. Descendents of enslaved people, for example, may identify themselves as African American because they are unable to trace their roots to a specific place.

Many areas have truly mixed populations, with people of several different ancestries nearly equally represented.

Take this area just southwest of Houston, for example:

Nigerian, Jordanian, Mexican, Vietnamese, African American, Salvadoran, Iraqi, German, English, Irish and Chinese people are all among the top groups in these neighborhoods.

Every city has its own distinct pattern, visualized in the the patchwork of gold, green and blue in Los Angeles, the stark reds, blues and yellows of Chicago, a purple Minneapolis, a green Honolulu:

Who comes next?

If the patterns in these maps reflect the immigration policies of at least a century ago, we can expect them to shift and change again as a result of contemporary decisions about who makes up the American mosaic.

No comprehensive immigration legislation has passed Congress since the 1980s. After a surge of immigration during the Biden administration, in which an estimated eight million people entered the country over three years, demographic experts now estimate that the United States could reach net-zero or negative immigration sometime soon. That is in part because of the Trump administration’s aggressive actions to speed deportations of people who are in the country illegally and to limit pathways to legal immigration.

At the same time, the factors that pull immigrants to the United States remain strong. And, unlike 100 years ago, the country now faces a declining population and work force. The tension between the need for new workers and resurgent nativist politics will influence who comes, who settles and who is counted among the ancestors of future generations.