Simplify the following text for a Grade 8 native English-speaking person.
Original: During the 1924 British Mount Everest Expedition , Mallory and his climbing partner Andrew `` Sandy '' Irvine both disappeared somewhere high on the North-East ridge during their attempt to make the first ascent of the world 's highest mountain .
Simplified: Mallory and his climbing partner Andrew Irvine both disappeared somewhere high on the North - East Ridge in an attempt to get to the top .