Complex sentence: The Khumbu glacier that forms the icefall moves at such speed that large crevasses open with little warning, and the large towers of ice (called "seracs") found at the icefall have been known to collapse suddenly.
Split into simpler sentences: The Khumbu glacier that forms the icefall moves at such speed that large crevasses open with little warning. The large towers of ice or "seracs" found at the icefall have been known to collapse suddenly.