What is the origin or reasoning behind calling someone inside an organisation feeding information to people outside it a mole?
In the OED there are many entries for the noun "mole". However in the one that relates to "small furry animals", in extended use there is included the figurative use of the word to mean "someone who works underground" e.g. a spy.
Note that in general sense (3a) the figurative idea has been around since the time of Shakespeare, and was used by the bard himself. However, where it relates to membership of an organisation dedicated to espionage and the security defences of a state (sense 3b), it is of far more recent coinage - and is said to have been rare before the time of the cold war. In literature it is heavily associated with the great espionage novelist of the period, John Le Carré.
The word IS NOT SYNONYMOUS with traitor. Mole refers to the type of work, and could just as easily be applied to a patriot as a traitor.
This is the full entry for sense 3.
II. Extended uses. 3. a. A person who works underground; a person who works in darkness or in secrecy.
1601 A. Dent Plaine Mans Path-way to Heauen 86 I wonder..yt these moulds [1603 moule] & muck-wormes of this earth, should so mind these shaddowish things [sc. riches].
1603 Shakespeare Hamlet i. v. 164 Well said old Mole, can'st worke in the earth? so fast, a worthy Pioner.
1622 Bacon Hist. Raigne Henry VII 240 Hee had such Moles perpetually working and casting to undermine him.
1745 E. Young Consolation 49 The Miser earths his Treasure; and the Thief, Watching the Mole, half-beggars him e'er Morn.
1855 J. R. Leifchild Cornwall: Mines & Miners 151 The miners there must have been generations of human moles pursuing their slow but certain advances in mysterious candlelight.
1990 D. Potter Hide & Seek (BNC) Four or five hundred yards from the black hole [sc. a mine entrance] the trees on the slope had sagged towards each other, disturbed by the human moles working beneath their long roots.
b. A penetration agent who over a long period achieves a position of trust within the security defences (esp. an intelligence agency) of a state; (more generally) a person who betrays confidential information from a position of trust within an organization, esp. over a long period. Cf. sleeper n. 2d, mole v.2 2b.Rare before writings on Cold War espionage in the 1970s; earlier uses appear to be isolated and lack the specificity of meaning which the term acquired in such writings. The term was popularized through the novels of ‘John le Carré’ (see quot. 1974); it is generally thought that the world of espionage adopted it from le Carré, rather than vice versa. For a detailed examination of possible origins of the term, see H. Cooper & L. Redlinger Catching Spies (1988) pp.187–248.
1922 Morning Post 28 Dec. 7/8 It is..necessary..to describe this document in detail, so that those who may be directly or indirectly affected by the underground burrowings of our Bolshevist moles will be familiar with their methods and plans.
1935 J. Buchan House of Four Winds xi. 234 I also have certain moles at my command... When the Cirque Doré mobilizes itself it has many eyes and ears. [1960 G. Bailey Conspirators (1961) vi. 124
[In 1935] ‘Ivanov’..displayed such a disconcerting knowledge of the innermost workings of the White military organizations that Fedossenko decided to join his network..in order to discover the source of his information. He was recruited under the alias of ‘The Mole’.]
1974 ‘J. le Carré’ Tinker, Tailor viii. 62 Ivlov's task was to service a mole. A mole is a deep penetration agent so called because he burrows deep into the fabric of Western imperialism.
1977 Time 11 July 10/3 He also introduced a secret computer system to ferret out even ‘sleepers’ and ‘moles’, deepcover agents whose meticulous disguises are planned for long-term use.
1980 National Times (Austral.) 10 Aug. 3/2 The death has sparked off speculation that Paisley was the mole long suspected of penetrating the CIA. 1990 D. Rutherford Game of Sudden Death (BNC) 283 We've had a tip from one of our moles in the Securitate.
2014 N. West Hist. Dict. Brit. Intelligence 350 She admitted passing classified material to the CPGB, including the name of Max Knight's mole inside the organization.
A mole, in espionage jargon, usually refer to:
a long-term spy (espionage agent) who is recruited before having access to secret intelligence, subsequently managing to get into the target organization.
however, it is popularly used to mean any long-term clandestine spy or informant within an organization, government or private.
Its etymology remains unclear, though it is probably based on the idea of “burrowing" as a metaphor for "one who works in darkness”. Its usage was made popular in a relatively recent book by Le Carré:
The term was introduced to the public by British spy novelist John Le Carré in his 1974 novel Tinker, Tailor, Soldier, Spy and has since entered general usage, but its origin is unclear.
Actually the are much older usage instances of mole meaning spy, dating, for instance, to the 17th century:
the term mole had been applied to spies in the book Historie of the Reign of King Henry VII written in 1626 by Sir Francis Bacon.