Simplify the following text for a Grade 9 native English-speaking person.
Original: In 1989 , the Great Dark Spot , an anti-cyclonic storm system spanning 13000 Ã -- 6600 km , was discovered by NASA 's Voyager 2 spacecraft .
Simplified: In 1989 , the Great Dark Spot , a cyclonic storm system the size of Eurasia , was discovered by NASA 's Voyager 2 spacecraft .