At the Mountains of Madness

As the two progress further into the city, they are ultimately drawn to a massive, ominous entrance which is the opening of a tunnel. Compulsively they are drawn in, finding further horrors: evidence of dead Elder Things caught in a brutal struggle and blind six foot penguins wandering around placidly. They are confronted with an immense, ululating horror which they identify as a Shoggoth. They escape with their lives using luck and diversion. On the plane high above the plateau, Danforth looks back and sees something that causes him to lose his sanity, and which he is too far gone to describe.

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That is not dead which can eternal lie.
And with strange æons even death may die.
--Abdul Alhazred's Necronomicon