All Blue

The All Blue is a mystical sea of legend, rumored to be the only place in the world where the North, South, East, and West seas meet. In this legendary ocean, it is said that there are fish from each of the four seas, and to have such a resource at one's disposal is the ultimate dream of every chef. Sanji strives to find this ocean of dreams. It is said that All Blue is found in the Grand Line. Zeff saved Sanji when he realized that they had the same dream of going to All Blue. In the 4Kids anime it is called the Great Blue. Although All Blue has yet to appear in the series, it was first mentioned in Chapter 56 and Episode 26, where Sanji and some chefs discussed if All Blue even exists.

In a Water 7 filler episode, Sanji finds another hint of the All Blue's existence. Salt from the All Blue is brought by the Aqua Laguna and is shown to be such a good seasoning that a small amount in food is enough to outclass even his own cooking. He later gathers all the All Blue salt he could and begins using it in all of his cooking. However, this cannot be considered canon.

Upon reaching Mermaid Cove in Fishman Island, Sanji exclaims to the crew that "he has found the All Blue there". However Usopp asks him whether he is certain, and it has not been confirmed whether Sanji had really found All Blue, and given the context the comment was most likely just an exaggeration, due to him being excited to see mermaids. However, due to Fishman Island being antipodal to Reverse Mountain as well as one of two connecting routes between Paradise and The New World, it is theoretically possible for the four seas to converge on that place.

Zoro also mentions that there are Eight seas when there're only seven, this was taken as him referring to the All Blue seemingly mocking Sanji.