Near the beginning of At the Mountains of Madness, the narrator explains that he is telling his story to discourage future expeditions to the Antarctic such as the planned Starkweather-Moore expedition. Beyond the Mountains of Madness is a campaign for the Call of Cthulu role-playing system containing the adventures of this new expedition.
The premise of the campaign is the ever-popular "the last expedition all died or went mad - we must find out what happened!" Following the disastrous Miskatonic University expedition to the Antarctic in 1930-31, the university began organising a second expedition, led by the famous explorer James Starkweather and William Moore, professor of geology. The players sign up for this expedition in whatever capacity suits them - the group I played with included a doctor with extensive cold-weather experience, a pilot, and a biologist - and, after various preliminaries, travel to Antarctica to follow in the footsteps of the previous expedition.
The published campaign is roughly the size of a small phone book, and is a major undertaking; it took my gaming group over a year of monthly sessions to get through it all. In addition to the plot of the adventure, it contains extensive source material on the Antarctic in general and the technology available to explorers in the mid-1930's. The preliminary stages can be rather slow, as the players are involved in acquiring and loading supplies for the expedition - complete cargo manifests and deck plans of the ship are provided - but once the investigators are on the ice the adventure builds rapidly.
As with any published campaign, the game master should read through it carefully with an eye to how the players are likely to actually react to the situations they face; our party managed to derail at least one subplot by taking what we considered quite reasonable steps, which the writers clearly had not anticipated. It's also worth noting that once the PCs are in Antarctica, they're in a "die or go forward" situation; even with the most modern wireless communications, the chances of being airlifted out of the depths of the eighth continent on short notice are nonexistent. Depending on the character of the players and the group as a whole, this could feel forced. My gaming group had a keeper who isn't afraid to improvise a bit when his players wander off script, and on the whole found the adventure engaging enough that it wasn't a problem.
Engan, Charles and Janyce, Beyond the Mountains of Madness, Chaosium Inc., 1999, ISBN 978-1568821382. This edition is out of print and hard to find. There are rumours of a second edition, maybe sometime next year, but I can't find anything concrete.