In the case of "At the Mountains of Madness" he used contemporary accounts of Arctic and Antarctic exploration to help fill in the details of the ill-fated Miskatonic expedition. In particular, Roald Amundsen's unsuccessful attempt to be the first to fly to the North Pole in 1925. ATMOM is filled with details drawn from the flight, from the way fuel was cached to the use of Dornier Wal's as the party's aircraft of choice.
To an extent, I'm following in Lovecraft's footsteps with my own "From the Mountains of Madness" project. Accounts of the Amundsen expedition have been a valuable source for photographs, text, and documents in my attempt to recreate the historical record of the Miskatonic expedition to the Antarctic. That's why I was excited to discover that someone has uploaded video transfers of the old Pathe newsreels of the Amundsen adventure to YouTube.
The quality isn't the best, but what can you expect from film that was shot over eighty years ago?