Columbia subject headings:
Harvard subject headings:
Intelligence service--Great Britain--History--20th century--Sources.
National Archives (Great Britain)
Submarines (Ships)--Germany--History--20th century--Sources.
World War, 1914-1918--Naval operations--Submarine
Princeton subject headings:
Submarines (Ships)--Germany--History--20th century.
World War, 1914-1918--Naval operations--Submarine.