unfathomable

/ʌnˈfæðəməbəl/
adjective
  1. Impossible to understand or comprehend; extremely mysterious or deep.
    • The universe is so vast that its size is unfathomable to the human mind.
    • The complexity of the human brain remains unfathomable even to scientists.
    • She stared into his unfathomable eyes, unable to guess what he was thinking.
  2. Impossible to measure the depth of (used literally for bodies of water or cavities).
    • Divers avoided the unfathomable cave, fearing they would never reach the bottom.
    • The unfathomable ocean trench plunged miles below the surface.
    • Ancient sailors believed the unfathomable abyss was home to sea monsters.
What does "unfathomable" mean? | whatsthatwordmean | whatsthatwordmean