His 30 April 1990 ascent of Cho Oyu, which he completed alone in low visibility, is disputed by some observers. Cho Oyu has a broadly flat summit plateau with no cairn (the traditional prayer flags on Cho Oyu's summit plateau do not mark the technical summit). The summit is a small unmarked hump (or bump) (which many Cho Oyu YouTube summit videos miss). While the height differential of this hump is small, Ralf Dujmovits, 3-time Cho Oyu summiter, notes that for a strong climber to get to the hump area can take another 30 minutes.