Antonyms of zenith:
nadir, abyss, base, foot, minimum, bottom, rock bottom.
Examples of usage:
  1. Madame Titjiens was no longer at the zenith of her great power, but she was very fine for all that.
    "Memoirs of an American Prima Donna", Clara Louise Kellogg.
  2. The west, half- way to the zenith, was green- the black- and- blue green of bruised flesh.
    "Roof and Meadow", Dallas Lore Sharp.
  3. Over it, through it, and around it, rolled the white fog, in great masses, caressing the earth and hanging from the zenith, like the veil of the temple of the Most High.
    "Half a Century", Jane Grey Cannon Swisshelm.