Simplify the following text for a Grade 9 native English-speaking person.
Original: The surface of the cerebral cortex is folded in large mammals , such that more than two-thirds of it in the human brain is buried in the grooves , called `` sulci '' .
Simplified: The surface of the cerebral cortex is folded in large mammals , such that more than two-thirds of it in the human brain is buried in the grooves .