Antonyms of apex:
antapex, abyss, base, foot, minimum, bottom, nadir, rock bottom.
Examples of usage:
  1. When the moon came up an hour later Red Connors, lying full length on the apex of the pinnacle which Johnny had tried and found wanting, watched an indistinct blurr of men in the shadow of the mesa wall.
    "Hopalong Cassidy", Clarence E. Mulford.
  2. Our line here makes a sharp bend round the corner of an orchard, and for security's sake a second trench has been cut behind, making, as it were, the cross- bar of a capital A. The apex of the A is no health resort.
    "The First Hundred Thousand", Ian Hay.
  3. The apex of the triangle of dark- blue sky was growing brighter and bluer, but he did not realize as yet that this was because he was coming nearer to the lights of the great house and garden.
    "The Wisdom of Father Brown", G. K. Chesterton.