SOFIA (AFP) A Bulgarian climber has died on an expedition to the top of the worlds second-highest mountain, K2, in Pakistan, a Bulgarian climbers website climbingguidebg.com reported Tuesday. Petar Unzhiev, 40, was part of an 18-man expedition to the 8,611-metre peak, which is considered the most difficult to climb. Unzhiev died while ascending the peak, Adventure Tours Pakistan confirmed to the website, saying altitude sickness was the cause of death.