an antonym for that would be at the mountains base
crevasse, valley
a valley is the opposite of mountain.
valley
mountain
the peak of the mountain
The opposite landform could be a lowland. Or in the ocean, a trench, deep or abyss. In mountains, the natural opposite of a peak is a valley. The opposite structure of a mountain, where the elevation is lower rather than higher, is a crevasse or chasm.
There are many possible words based on the meaning of "plain" that is used.The word pretty used a woman's appearance could be the opposite of plain.The word peak used for a mountain would be the opposite of a plain or prairie.The words pretentious, pompous, or poshcould be the opposite of a plain or ordinary event.The word private could be the opposite of something plain, as in obvious.The word phenomenal could be the opposite of a plain or mundane experience.The word prodigious could be the opposite of a plain performance.
I'm assuming that you would be referring to the "peak" of a mountain. The peak of the mountain is generally the tallest point; thus opposite of the base.
A gorge.
the oppposite OS the base of a mountain is the summit
The Peak
The type of mountain that forms when forces pull from opposite directions is the fault-block mountain.
what word is opposite of prudent
The opposite of anything is it's absence. Silence would be the opposite of a word spoken.
The opposite of the base of a moutain would be it's peak. The very tip of the mountain where all the snow rests and where it's coldest.