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"Swallow Blue" is Einar Skinnarland, a native of Rjukan, the town nearest the plant. From his hut on the Hardanger Plateau above Rjukan, Skinnarland, along with other members of the Resistance both outside and within the plant, served as the Special Operations Executive's eyes on Vemork. "IMI" is the codename for heavy water. By "reprisals" after an attack, Skinnarland means the Germans would naturally suspect and take revenge on local Norwegians, a great concern to the Resistance fighters, who grew up in the area.