Fo Halloo (Manx: "Underground") was a militant Manx nationalist group active on the Isle of Man in the 1970s. The group conducted Manx graffiti and poster campaigns, published and distributed newsletters, and was also accused of conducting a number of arson attacks against the homes of English, non-Manx residents.
An inference that the Celtic League was involved in the "Manx underground" activities was published by The Guardian in 1976; a refutation from the League chairman was eventually published a month later after ongoing efforts by the League.
- John Belchem (1 May 2001). New History of the Isle of Man Vol. 5: The Modern Period, 1830-1999. Liverpool University Press. pp. 11–. ISBN 978-0-85323-726-6. Retrieved 14 August 2012.
- Peter Berresford Ellis (15 December 2002). Celtic Dawn. Y Lolfa. pp. 157–. ISBN 978-0-86243-643-8. Retrieved 14 August 2012.