On an expedition to Antarctica, Professor William Dyer and his colleagues discover the remains of ancient half-vegetable, half-animal life-forms. The extremely early date in the geological strata is surprising because of the highly-evolved features found in these previously unknown life-forms. Through a series of dark revelations, violent episodes, and misunderstandings, the group learns of Earth's secret history and legacy.
At the Mountains of Madness is a very good introduction to Lovecraft's mythology of the Old Ones and the glories and horrors that preceded humankind and that still linger in places. It is a good novella on it's own, and helpful to understanding events and creatures in his other works.
This new edition of Lovecraft's masterpiece includes two essays "Notes On Writing Weird Fiction" and "Supernatural Horror in Literature".