After more than 3,000 Marines were killed in the Battle of Tarawa (November 1943), it became clear that the U.S. military was in need of better pre-invasion intelligence. Enter the Naval Combat Demolition Units and Underwater Demolition Teams (UDTs), the forerunners of today’s SEALs. After World War II, however, these special operations forces largely disbanded. But beginning in 1950, with the outbreak of the Korean War the Navy called on the UDT “frogmen” again, and quickly expanded their operation.

In 1962, as the conflict in Vietnam began ramping up, President John F. Kennedy established the first two Navy SEAL teams out of the existing UDTs. The SEAL acronym comes from Sea, Air and Land, the three environments where the Navy’s special operations forces are trained to operate. At the height of the conflict in Vietnam, eight SEAL platoons were deployed there on a rotating and continuous basis, and close to 50 SEALs were killed in Vietnam from 1965 to 1972.

In late 1980, after the humiliating failure of Operation Eagle Claw, the aborted mission to rescue 53 American hostages seized at the American embassy in Tehran, the Navy asked Commander Richard Marcinko to build a SEAL unit that could respond quickly and fiercely to terrorist crises. Marcinko was a seasoned veteran, having enlisted in the Navy in 1958. He served two tours in Vietnam, where he commanded a much-feared SEAL platoon, and earned the Silver Star, four Bronze Stars with combat “V” (denoting heroism), two Navy Commendation Medals and the Vietnamese Cross of Gallantry with Silver Star. He was reportedly given six months to get the new counterterrorism operation up and running, or the entire project would be scrapped.

Though only two SEAL teams existed at that time, Marcinko called the new group SEAL Team Six, supposedly because he hoped Soviet analysts would overestimate the size of the U.S. forces. Two assault groups, named after the Navy colors of blue and gold, formed the core of the group. The Blue Squadron, with the Jolly Roger pirate flag as its insignia, soon earned a reputation for recklessness, while the Gold Squadron identified more with knights or crusaders. Marcinko left after several years (he formed another anti-terrorist unit, Red Cell, in 1984, but in 1990 was convicted of military contract fraud and served 15 months in prison) and in the early 1990s the Navy reportedly stepped in to revamp Team Six’s leadership and operations, turning it into the professional and effective—yet still boundary-pushing—force it is today.

Officially, SEAL Team Six doesn’t even exist. As Dick Couch and William Doyle write in their 2014 book “Navy SEALS: Their Untold Story,” the U.S. Department of Defense almost never publicly acknowledges the existence of the Naval Special Warfare Development Group, or DevGru, the cover name for Team Six. Its official mission is developing new equipment and tactics for the general Navy SEAL organization, which also includes nine unclassified teams. Unofficially, however, SEAL Team Six carries out some of the military’s riskiest missions, the ones considered too dangerous for conventional troops.

Since the terrorist attacks of September 11, 2001, Team Six and the rest of the Navy SEALs have found themselves playing a more active role than ever, ranging from the remote, mountainous regions of Afghanistan to war-torn cities such as Baghdad. The SEALs, including Team Six, carry out clandestine, high-impact operations that would be impossible for larger, conventional forces. They also perform on-the-ground reconnaissance and intelligence gathering before planned attacks by those larger forces. Though traditionally SEALs were associated most with (at least partially) water-based missions, they are equally likely to carry out missions on land and in the air.

Three successful operations in recent years pulled the SEALS, and Team Six in particular, out of the shadows and squarely into the global spotlight. In April 2009, Somali pirates captured Captain Richard Phillips of the merchant ship MV Maersk Alabama and held him hostage inside a small, enclosed lifeboat. The American destroyer USS Bainbridge was towing the boat to calmer waters in the Indian Ocean when ransom negotiations stalled, and the three SEAL Team Six snipers on the warship shot and killed the three pirates holding Phillips. Details of the rescue made international news and formed the basis for a major Hollywood film, “Captain Phillips,” starring Tom Hanks. In January 2012, Team Six operators skydived into Somalia to save two hostages, American aid worker Jessica Buchanan and her Danish colleague Poul Thisted.

By far the highest-profile Team Six operation—and the most famous special ops raid in history—was Operation Neptune Spear, which ended in the killing of Al Qaeda mastermind Osama bin Laden in Abbottabad, Pakistan in May 2011. The culmination of a 10-year manhunt directed by the Central Intelligence Agency (CIA), the raid on bin Laden’s compound was carried out by 23 or 24 SEALs (according to varying accounts), accompanied by a Pashto translator and a combat dog. It took less than 40 minutes.

As the role and importance of SEAL Team Six has expanded greatly since 9/11, so has the danger. As the New York Times reported in 2015: “More members of the unit have died over the past 14 years than in all its previous history. Repeated assaults, parachute jumps, rugged climbs and blasts from explosives have left many battered, physically and mentally.”

Today, the top-secret headquarters of SEAL Team Six are located at the Dam Neck Annex of the Oceana Naval Air Station, just south of Virginia Beach. Elite operators from regular SEAL teams are chosen to join Team Six in a competitive process known as “Green Team.” Two more assault groups, Red Squadron and Silver Squadron, have joined the Blue and Gold, for a total of some 300 operators in all. Members of the Grey Squadron, known as the vikings, are trained specifically to drive the high-speed boats and other vehicles used by Team Six, while the Black Squadron, which began as Team Six’s sniper unit, has taken the lead in gathering intelligence since the 9/11 attacks. Women—who are excluded from the rest of Team Six—can serve in the Black Squadron, which is estimated to have some 100 members stationed throughout the world.