Definition of "unfathomed"

Youth is not less averse from change; but that is from exaggeration of its consequences, for all seems to the young so important, and so fatal. They are timid, because they know not what they fear; hopeful, because they know not what they expect. Despite their gayety of confidence, they yet dread the first plunge into life's unfathomed deep.

1837, L[etitia] E[lizabeth] L[andon], “Age and Youth”, in Ethel Churchill: Or, The Two Brides. […], volume I, London: Henry Colburn, […], page 8

As she told me, I was naught to her, and never shall be through the unfathomed depths of Time, unless, indeed, conditions alter, and a day comes at last when two men may love one woman, and all three be happy in the fact.

1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887

Man does vvith dangerous Curioſity / Theſe unfathom'd VVonders try: / VVith fancy'd Rules and Arbitrary Lavvs / Matter and Motion he reſtrains, / And ſtudy'd Lines and fictious Circles dravvs; […]

1688, Mat[thew] Prior, “On Exodus 3. 14. I Am that I Am. An Ode. […]”, in Poems on Several Occasions, 2nd edition, London: […] Jacob Tonson […], published 1709, stanza VI, page 4