Opposite words for apex:
base, antapex, rock bottom, minimum, nadir, foot, bottom, abyss.
Usage samples:
  1. The figure is invariably that of a triangle, with its base on the white of the eye and its apex more or less advanced over the cornea toward its center.
    "Special Report on Diseases of Cattle", U.S. Department of Agriculture J.R. Mohler.
  2. In fact, the aberration is really equivalent to drawing each star from its mean place towards the Apex of the Earth's Way, as the point is sometimes termed.
    "The Story of the Heavens", Robert Stawell Ball.
  3. One of these is of triangular shape about 150 feet wide at the base and 80 feet in height to the apex.
    "The Tribes and Castes of the Central Provinces of India Volume II", R. V. Russell.
  4. Here its base shall rest and its apex point to the heavens through the coming centuries.
    "Reminiscences of Sixty Years in Public Affairs, Vol. 1", George Boutwell.