 Complete the following phrase:  On 18 April 2014, a serac collapsed above the Khumbu Icefall on Mount Everest, causing an avalanche of massive chunks of ice and snow which killed 16 Nepalese guides, mostly Sherpas. The 2014 avalanche is the second-deadliest disaster in Everests history, only exceeded by avalanches in the Khumbu Icefall area a year later, on 25 April 2015, caused by a 