Simplify the following text for a Grade 6 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 . The storm resembled the Great Red Spot of Jupiter .
Simplified: Neptune once had a huge storm known as the `` Great Dark Spot '' which was discovered in 1989 by Voyager 2 .