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The noun iceberg designates a large mass of ice floating in the sea.

Floating ice has about 88 per cent of its mass submerged. This property has given rise to the figurative use of the noun iceberg in the sense of something of which the greater part is unknown or unrecognised—especially in the phrase the tip of the iceberg and variants, which designate the smaller, perceptible part of something which is evidently much larger.

(There is no evidence that this figurative use was (even partly) motivated by the sinking, on Monday 15^th April 1912, of the R.M.S. Titanic after a collision with an iceberg.)

A variant of the phrase the tip of the iceberg occurs, for example, in the following from No sports lull at Christmas, by Frank Gianelli, published in The Arizona Republic (Phoenix, Arizona, USA) of Tuesday 14^th December 1971 [page 51, column 2]—“that year” refers to 1951:

Lloyd Mangrum that year was the leading money winner on the limited Professional Golfers’ Association trail—earning a headlined $26,088. Jack Nicklaus this year set a new record with $249,000—and that’s just what shows on the tip of an earnings iceberg which has an underbase of personal engagements, product endorsements, sideline businesses.

These are, in chronological order, the earliest occurrences that I have found of the phrase the tip of the iceberg and variants used in metaphors and in similes:

1-: From Workings of the New Method for Handling Nervous Children: The Case of Ralph, by the U.S. poet, novelist and psychoanalyst James Oppenheim (1882-1932), published in Harper’s Bazar (New York City, New York, USA) of October 1912 [page 486, column 3]—the fact that the author specified “to use the well-known symbol” seems to indicate that the tip of the iceberg was already in common usage:

The whole point is, that the mind never forgets. This may sound strange in the face of the fact that we seem to be forgetting all the time. But what actually happens is this: what you have “in your mind” at any given moment is but a minute part of what is really in your mind; in other words, to use the well-known symbol, the conscious part of the mind is like the tip of the iceberg that shows above the water-line, and the unconscious part like the huge bulk of ice beneath.

2-: From Sentimentality and Social Reform, by the U.S. sociologist Arthur James Todd (1878-1948), of the University of Minnesota, published in The American Journal of Sociology (The University of Chicago Press) of September 1916 [page 159]:

The tragedies of leadership in social reform result in the main from failure to work out a practicable basis of partnership between ideas and sentiments. Men recognize theoretically that ideas always appear swaddled in feelings. But many of us go about the day’s business apparently on the assumption that ideas are as clear-cut and unemotional as hammers or rifles. Hence our projects fail to capture men’s hearts and imaginations. We have to recognize that after all reason in men is only the very tip of their iceberg of mental life. We live by our sentiments, even by our illusions. They furnish the real motive power which makes things go. They are at the bottom of our choices.

3 & 4-: From Far Trouble (London: W. Collins, [1931]), a story set in China, by the U.S. novelist Thomas Bowyer Campbell (1887-1976):

3-: [page 25]:

“China is like an iceberg,” he said, “the greater part and the dangerous part is under water, out of sight. Shanghai is but the crystal tip . . . so fine to see in the sunlight. But under the surface . . . Shanghai is not China.”

4-: [page 28]—the following is from a description of the Shanghai Bund:

Everywhere policemen, marines, and armed civilians . . . at all street-corners, before gateways and doors, even on terraces and balconies . . . order and protection for the International Settlement . . . the tip of the iceberg out of the water, underneath the murderous, jagged spars . . . waiting.

5-: From Improving Alumni Relations: Have the Colleges and Universities Met the Problem?, an address delivered at the Twenty-Fifth Meeting of the American Alumni Council, Columbus, Ohio, on Saturday 2^nd April 1938, by J. L. Morrill, published in The Quarterly Review of the Michigan Alumnus (Ann Arbor, Michigan, USA) of Summer 1938 [page 359, column 2]:

The alumni secretary is usually a capable, enthusiastic, and useful member of the staff. Like the superintendent of buildings and grounds, he is efficient. At Commencement time there are enjoyable reunions and fine parades. The alumni do give some money, and they have local club meetings all over the country with more speaking invitations to the president than he can find time to accept. He sees the glowing sunlit tip of the iceberg. Only the alumni secretary, in the clairvoyance of his more despondent moments, senses the chill of its huge and submerged segment.