unplumbable

When you're borrowing from the future, the well is literally unplumbable.

Keith Blanchard: I'll Take MY Bailout, Now, Thanks! 2008

Since to Sergei women were all an unplumbable mystery, most especially Katerina, whose beauty made it impossible for men to think around her, he had no intention of trying to understand anything except what his errand was supposed to be.

Enchantment Card, Orson Scott 1999

A few ships burned and a few were still sinking, but most had sunk into the unplumbable depths of the channel.

Elric of Melnibone Moorcock, Michael, 1939- 1972

The eyes were a volcanic blue, like twin pockets of unplumbable depth.

Conan the Avenger Howard, Robert E. 1968

She looked smooth and precious, a thing of unplumbable mysteries, and perilous as quicksand.

Cider With Rosie Lee, Laurie 1959

But somehow, from the unplumbed and unplumbable depths which made her what she so uniquely was, the Red Lensman drew more and ever more power.

Children of the Lens Smith, E. E. 1954

Vast valleys snarled into life; gorges sheered off into unplumbable depths, and then leaped up ferociously in ugly snags of mountain.

Slan Van Vogt, A. E. 1946

All of them came from without -- or from unplumbable and uncontrollable depths within.

Damn! A Book of Calumny 1918

But here he started and turned to stare toward the windows as from the darkness beyond two voices were uplifted in song; two voices these which sang the same tune and words but in two different keys, uncertain voices, now shooting up into heights, now dropping into unplumbable deeps, two shaky voices whose inconsequent quaverings suggested four legs in much the same condition.

The Definite Object A Romance of New York Jeffery Farnol 1915

I was a bit nervous, I suppose, but no one could be really scared or alarmed in the unplumbable depths of calm in which that British major was plunged!

A Minstrel in France Harry Lauder 1910