Early in 1961, teams of United States Special Forces units began arriving in South Vietnam to train the Vietnamese in counterinsurgency. The initial Special Forces teams were army units, but the United States Navy also started its own counterinsurgency groups, known as sea, air, and land (SEAL) teams. The first SEAL groups arrived in South Vietnam in 1966 and went into the swamps of the Mekong Delta. They engaged in an array of activities. SEAL teams infiltrated Vietcong areas in small boats or as frogmen, parachuted in or landed by helicopters, conducted "hunter-killer" raids, worked with Studies and Observation Groups intelligence teams, and operated from fast-moving airboats and Seawolf helicopters. The SEAL were supported by naval Task Force 116.