Together with Simpson, Yates climbed Siula Grande in 1985, via the hitherto unclimbed west face. On the descent, Simpson fell through a cornice, breaking his right leg and heel. To continue descending, Yates then used ropes to lower Simpson down the mountain in stages. While descending in the night in bad weather, Yates lowered Simpson over an unseen cliff edge, which meant that he was hanging over a deep crevasse with only Simpson's hold on the rope to prevent him falling. To avoid being pulled off the mountain himself, Yates cut the rope. Simpson thus fell approximately 50 feet into the crevasse. He survived the fall, unbeknown to Yates, who assumed he must have been killed. Simpson managed to crawl out of the crevasse and reached base camp four days later.