In May 2011, Lock attempted Everest for the third time, but his first without supplementary oxygen (to complete the rarer Reinhold Messner feat of climbing all 14 eight-thousanders without oxygen). His solo climb of Everest's North Ridge was unsuccessful due to high winds and blizzard conditions. Lock made a second attempt to summit Everest solo, via the North side, without supplementary oxygen in May 2012, but abandoned the climb 300 metres from the summit due to self diagnosed early symptoms of Cerebral Oedema. [This was incorrectly reported as High altitude pulmonary oedema].