The Department of Homeland Security has an imperfect count of how many foreign visitors to the U.S. overstayed their visas on tourism, business and other trips. Immigration officials say they haven’t had the resources to track the entries and exits of these authorized visitors, who are awarded what DHS calls “nonimmigrant visas.” The agency has released its best guess for fiscal year 2015 (fall to fall), and the numbers are eye-popping. Below, we list 7 important statistics for understanding visa overstays, and the ongoing calls for an exit/entry system.

45 Million Visitors: The DHS reports 45 million international travelers touched down in the U.S. and were expected to depart in 2015. The agency argues that around 99 percent leave the country. The average overstay rate is 1.07, and slightly higher for visitors from Canada and Mexico.

482,781 Overstays: An estimated half a million migrants overstayed their visas in the 2015. The top offending countries in raw numbers were Canada (93k), Mexico (42k), Brazil (35k) and Germany (21k). Refugee-sending countries like Afghanistan, Chad and other countries had some of the highest rates of overstays.

180 Days: Visitors to the U.S. start getting in serious trouble after they stay in the country without permission for 6 months. After that point, tourists, business people and other temporary visitors can face a 3 year ban from the U.S., and a ten year ban if the stay for over 365 days.

40 Percent: the approximate share of the estimated 10.9 million immigrants living in the country illegally who entered the country on a visa and overstayed.

1997 C.E. : The year that immigration officials (then part of INS) were tasked with creating an “exit-entry” system. In testimony to a U.S. Senate Judiciary Committee hearing this week, a senior DHS official estimated that such a system might take another decade to complete. He was met with bipartisan ire.

$3 Billion: The estimated cost of retrofitting airports to accommodate a thorough exit system a according to the New York Times, who cited a 2013 DHS estimate. Inspections at land crossings are not required by Border Patrol and are performed sporadically.

"This is the very essence of open borders," Sen. Jeff Sessions (R-Ala.) said. "Anyone can come in, no one has to leave."

2 Hours: The estimated wait time added to international flights if an exit system were implemented, according to DHS testimony in 2015. One barrier cited by the official was space in the airports.

"This is about national security,” Sen. Al Franken, (D-Minn.) said, “and it's hard for me to envision that we can't figure out where to get space to do this at an airport or seaport."