unfathomable

adj

Etymology: From un- (prefix meaning ‘not’) + fathom (“to measure the depth of (water); (figurative) to deeply understand (someone or something)”) + -able (suffix forming adjectives denoting things not able or fit to be done).

1. Impossible to fathom.

   “[W]e are not to make our eſtimate of the quantity of VVaters meerly by the Superficies of the Sea, but by its vaſt depth, vvhich in ſome places is unfathomable, and by thoſe vaſt ſubterraneous Receptacles of VVater vvhich pour themſelves out in ſeveral great Ebullitions and Marine Springs: […]”

   “But if we yet rise higher, and consider the fixed stars as so many vast oceans of flame, that are each of them attended with a different set of planets, and still discover new firmaments and new lights, that are sunk farther in those unfathomable depths of ether, so as not to be seen by the strongest of our telescopes, we are lost in such a labyrinth of suns and worlds, and confounded with the immensity and magnificence of nature.”

2. Impossible to fathom.

   “The sheer number of warriors the enemy attacked with was unfathomable.”

   “There are indeed unfadomable depths in that Ocean [i.e., the holy scriptures], vvherin vve ſhall vainly hope to pitch our anchor; but all neceſſary truthes need not much line: […]”