fathomless

/ˈfæðəmləs/
adjective
  1. Too deep to be measured; extremely deep.
    • They stared into the fathomless abyss of the canyon.
    • The lake appeared fathomless, its dark waters hiding unknown secrets.
    • The ocean in that region is fathomless, with depths no one has ever reached.
  2. Impossible to understand or comprehend fully; mysterious.
    • The universe remains fathomless in its complexity.
    • He gazed at the painting with fathomless wonder, unable to grasp its meaning.
    • Her eyes held a fathomless sadness that no one could explain.