One theory is that the aircraft was blown off course by strong winds.
Most of the passengers were from South Korea.
The flight from Beijing was just coming in to land but crashed in mountains near a housing area.
But thick smoke and bad weather have made it really hard for rescue workers.
They have found the plane's black box though which will give them clues about what happened.
World Cup worries
The crash comes just weeks before South Korea holds the World Cup football finals with Japan.
Because of this, there have been more flights into South Korea for the thousands of visitors who are expected to come to the matches.
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