Opposite words for zenith:
nadir, abyss, minimum, base, rock bottom, foot, bottom.
Usage samples:
  1. Brimfield had met Southby Academy in the afternoon and had torn the visitors to tatters, scoring almost at will and sending the hopes of her adherents soaring into the zenith.
    "Left End Edwards", Ralph Henry Barbour.
  2. Now, do you see the two stars about half way between the horizon and the zenith?
    "A Victorious Union SERIES: The Blue and the Gray--Afloat", Oliver Optic.
  3. The sun is at the zenith.
    "Atlantida", Pierre Benoit.
  4. It would have been well for Louis Napoleon if at the zenith of his imperial success he had studied that message of President Monroe and properly interpreted its meaning.
    "A History of the Four Georges and of William IV, Volume IV (of 4)", Justin McCarthy and Justin Huntly McCarthy.