Complex sentence: They witnessed a marathon being held there, always won by the Sherpas due to them being accustomed to the thin air at that altitude and were under scrutiny on the Tibetan border by the Chinese army as their helicpoter teetered marginally into Chinese airspace.
Split into simpler sentences: They witnessed a marathon being held there, always won by the Sherpas due to them being accustomed to the thin air at that altitude. They were also under scrutiny on the Tibetan border by the Chinese army as their helicopter teetered marginally into Chinese airspace.