Hillary forged a route through the treacherous Khumbu Icefall. Other natural causes include rain, earthquakes, rockfall and icefall. The deaths were caused by an ice-fall avalanche in the Khumbu Icefall. This behavior often poses the biggest risk to mountaineers climbing in an icefall. Since the structures are continually changing, crossing the Khumbu Icefall is extremely dangerous. Twenty-six porters for the Khumbu Icefall were led by their sirdar Phurkipa. The most dangerous time to cross the Khumbu Icefall is generally mid- to late-afternoon. This was the same icefall where the 1970 Mount Everest disaster had taken place. Above the icefall is Camp I at . The "Icefall party" reached Base Camp at 17,900 ft (5455 m) on 12 April 1953. The team used SoftSun lights for creating sun on the summit, Hillary Step and icefall. Right of "The Icefall" is "The Lip". Furthermore, Solveig is the main character and narrator of Matthew J. Kirby's "Icefall". The gamble at the icefall succeeded, and the men reached their depot two days later. However, the flow of ice in an icefall may be measured in kilometres per year.