It’s probably America’s most controversial list. You can be put on it without your knowledge, and getting off it is extremely difficult.

It’s the federal no-fly list, a collection of names of people who are not allowed to board commercial flights into or out of the United States. According to leaked documents obtained by The Intercept, more than 47,000 people were on America’s no-fly list as of August 2013. That number reportedly includes 800 Americans, many of whom don’t even know they’re on it. The government sends no official notification to those on the list; many times, people don’t find out until they’re denied boarding at the airport.

A number of high-profile lawsuits have claimed the government unjustly added people to the list and blocked their efforts to have their names removed.  Last summer, a federal court ruled in favor of 13 people who claimed the government violated their constitutional rights to travel by placing them on the no-fly list. The government was ordered to tell the plaintiffs whether they’re on the list, spell out the reasons they are barred from travel, and to give them a chance to challenge the government finding.

The case was one of the biggest challenges yet to the super-secret government list, but the veil that shrouds the no-fly list still remains. 

“There is this black box procedure which operates purely behind the curtains and no one is able to part those curtains and find out what really goes on,” airline industry analyst Robert Mann explained to Yahoo Travel.

The government maintains that the secrecy about the no-fly list is necessary for national security. A potential terrorist can be tipped off that the government is watching him, the logic goes, if he gets a letter saying he’s now on a government blacklist.  

“I think it’s like any high security process,” says Mann. “You don’t want to reveal sources and methods because knowledge of those sources and methods permit nefarious individuals to compromise them.”

Despite the secrecy, various court cases, news reports and leaked documents have shed some light on the process behind the no-fly list. Here are eight possible ways one can end up on it.