Editor's note: since production of the map, most media organizations have begun referring to this population as "undocumented" rather than 'illegal" immigrants.

What else do we know about undocumented immigrants in the U.S.?

The population has actually gone down quite a bit since 2007, when it spiked at about 12 million, according to Pew. The decrease is due largely to the U.S. recession and increased border enforcement and deportations, with the rate of undocumented immigration from Mexico falling the most.
Mexicans made up close to 60 percent of all undocumented residents, according to a Pew analysis of the 2010 population. DHS estimates that in 2011, 70 percent of this population came from Mexico, Guatemala and El Salvador.
Today's estimated 11.1 million undocumented immigrants make up less than a third of all foreign-born residents in the U.S. Roughly 40 to 50 percent of the undocumented entered the country legally and overstayed their visas, according to a Pew 2006 analysis. And although about 1.6 million of the total undocumented population today arrived within in the last years , the majority of the current population has lived here for at least a decade, reports the Department of Homeland Security reports.

So how do we know all this?

Counting America's undocumented population is a true exercise in estimation. Pew, a non-partisan public policy group, came up with the latest 11.1 million figure (for 2011) primarily by analyzing census data, which provides a measure of the total immigrant population (both legal and undocumented). Pew then analyzed a variety of other government data sources - including DHS - to estimate the number of legal immigrants (green card holders and refugees) and then subtracted this figure from the total number of immigrants. Of course, a lot more statistical wizardry goes into the calculation (as described here) but this is the basic framework for its estimation.

immigration reformimmigration reform

U.S. Immigration Reform Explained

11 Million Strong: Counting America's Undocumented Immigrants

Interactive Timeline: History of Immigration in America