http://en.wikipedia.org/wiki/Communication_with_submarines#Extremely_low_fr=
equency

VLF (10s of KHz) was used for subs beginning in the 1930s.

A US VLF site which still exists, but is no longer operational :

http://hawkins.pair.com/nss.shtml

A similar RN VLF site (GBR?) was shut down perhaps 10 years ago.
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