The fictional plateau associated with the exploration in Lovecraft’s “At the Mountains of Madness” is Leng, usually called the Plateau of Leng. The novella’s Antarctic explorers discover an immense abandoned city built by the Elder Things, and the text connects that city with the mysterious plateau known from Lovecraft’s wider mythos.
The expedition is led by William Dyer of Miskatonic University. After Professor Lake’s advance party discovers ancient, apparently fossilized organisms, Dyer and Danforth fly over a mountain range and find a vast city whose murals reveal Earth’s prehuman history. The Elder Things arrived on Earth from space, created shoggoths as servants, and fought other alien beings including Cthulhu’s spawn and the Mi-go.
Leng is easy to mix up with Kadath, Yuggoth, and Celephaïs. Kadath is a mountain in the Dreamlands, Yuggoth is generally identified with Pluto in Lovecraft’s fiction, and Celephaïs is a Dreamlands city. The novella’s Antarctic setting gives Leng a particularly ambiguous geographic identity, but the Plateau of Leng is the intended mythos answer.
Lovecraft wrote the novella in 1931. It was serialized in Astounding Stories in three installments from February through April 1936.