Simplify the following text for a Grade 8 native English-speaking person.
Original: Ishmael soon learns that Ahab seeks one specific whale : Moby Dick , a ferocious , enigmatic white sperm whale . In a previous encounter , the whale destroyed Ahab 's boat and bit off his leg .
Simplified: Ahab is seeking revenge on Moby Dick , a great white whale ( believed to be a Sperm Whale ) which has bitten his leg off .