Neptune is the farthest major planet from the Sun,and difficult to study. Even though it's big, approximately 50000 kilometers in diameter, or it's just so far away that details can be missing. Heck,even going there, like we did with Voyager 2 in 1989, and didn't solve all its mysteries.
But it did provide clues for a big mystery that cropped up later: What's the deal with Neptune's dinky moon Hippocamp?
Source: blastr.com