Secret agents have a long history of embedding themselves into foreign nations through what intelligence experts have dubbed “sleeper cells.”

According to the oxford dictionary, by definition a sleeper cell is:

“A secretive group of spies or terrorist agents that remain inactive within a target population until ordered to act.”

Sleeper cells have a long international history which has in some cases reached even the United States, and with an estimated 20 currently lying dormant according to European counterrorism agencies, they are more relevant than ever.

In 2014 a whopping 71 percent of Americans believed there was an ISIS sleeper cell in America.

Below are three of history’s most recognized sleeper cells.

1. The Illegals Program

In 2010, a record 10 agents were identified to be Russian spies in an FBI investigation that the U.S. Justice Department dubbed, “The Illegals Program.”

Planted by Russian Foreign Intelligence, the spies attempted to build contacts with policymakers, academics, and other political influencers in hopes of gaining valuable intelligence.

The agents were stationed in and outside of major east coast cities like New York, D.C., Boston, and some even attended major universities like Seton Hall and New York University.

Many of the agents worked in tandem to court political higher ups and gain intelligence.

After being convicted, the 10 spies were eventually exchanged for four individuals (presumably American spies) who had been similarly imprisoned in Russia for espionage.

2. Kuehn Family

Just before the start of WWII, Bernard Julius Otto Kuehn was assigned, by Nazi Germany, to live as a sleeper agent in Honolulu, where he would provide the Japanese military valuable intelligence about the U.S. Navy.

Reports show that one of Kuehn’s sons was as young as 11-years-old when he was taught to ask pointed questions about U.S. naval vessels.

Kuehn, with his wife, daughter, and two sons, lived in Honolulu undetected until the attack on Pearl Harbor when U.S. military caught Kuehn flashing coded messages to Japanese spy Takeo Yoshikawa.

Kuehn was originally sentenced to death by the U.S. government, but his sentence was commuted to 50 years after providing crucial information on Nazi spy networks.

3. The Cambridge Five

The Cambridge Five were a group of British spies who delivered key intelligence to the Soviet Union during WWII. Kim Philby–whose life has been popularized by film classics like Tinker, Tailor, Soldier, Spy–was one of the more famous agents to come from the group.

The spies were said to have been recruited in 1930 when they attended Cambridge University. Philby, one of the most successful spies, even found himself embedded in the CIA by the late 1940s where he continued to supply the Soviet Union with information.

The four of the spies identities have been known for some time, but the fifth still remains somewhat mysterious to this day.

4. French Al Qaeda Operatives

Following the killing of 11 French magazine staffers, Cherif Kouachi and his brother Said were revealed to be Al Qaeda sleeper agents operating silently in France.

The two brothers–both natives of France–were radicalized over a period of time, starting in 2005 when Cherif was arrested for attempting to leave France for Syria to fight U.S. soldiers.

French authorities, who were given information regarding the brothers’ training in Yemen, monitored the two until Spring 2014–stopping just before their infamous attack.

In the events proceeding their assault on the staffers, the two sleeper agents were killed by French police.