Yannis Pitsiladis, scientist and provocateur, had come here for the same reason that pilgrims wheezing with bronchitis and emphysema have headed to this low-altitude divide between Israel and Jordan. He had come for the oxygen. ... A quarter-mile below sea level at the Dead Sea, where the barometric pressure is high, there is about 5 percent more oxygen to breathe. The naturally enriched air had been shown to increase exercise capacity in those with chronic lung disease. Would it do the same, Pitsiladis wondered, for the world’s fastest distance runners? ... He wanted to redefine the limits of human endurance by training a man to run a marathon in less than two hours without the use of performance-enhancing drugs. ... The Sub2 Project, as it is called, is an attempt at the extraordinary — to reduce by nearly three minutes the world record of 2 hours 2 minutes 57 seconds, set at the 2014 Berlin Marathon by Dennis Kimetto of Kenya. ... Some consider the goal impossible. Many are suspicious because of widespread doping in track and field, and almost no one considers the feat achievable anytime soon. ... To run under two hours without the use of banned drugs would be to set a record that would stand with the four-minute mile as an ultimate test of human stamina. ... A 1:59:59 marathon would require a searing pace of 4 minutes 34 seconds per mile, seven seconds faster than the pace of the current world record. It would require 85 to 90 percent of a runner’s maximum aerobic capacity — twice the capacity of an average man — and a sustained heart rate of about 160 to 170 beats per minute.