So we have all seen Zoro boast.
Over the 9 mountains and 8 seas... Throughout the world itself... There is nothing I cannot cut.
Now it made me wonder, which mountains and what seas was he referring to?
So we have all seen Zoro boast.
Over the 9 mountains and 8 seas... Throughout the world itself... There is nothing I cannot cut.
Now it made me wonder, which mountains and what seas was he referring to?
Some of Zoro's attacks are named after Buddhism like 36/72/108 Pondo Hou
the 36/72/108 Pondo Hou, is a reference to the 36 passions of Buddhism
So most likely Zoro is referring to the Buddhist legend of Sumeru the central world-mountain in Buddhist cosmology.
I'm not an Buddhism expert but in the Wikipedia article about Sumeru you see a list of 9 mountains and 8 seas, so since Zoro likes Buddhism and Hindu references in his attacks, it's most like Oda made a reference to this.
Mintri's answer is right, but for the sake of having an in-universe/joke answer:
8 Seas:
9 Mountains:
With the remaining 4 yet to be seen.
Nine mountains and eight seas [九山八海] ( kusen-hakkai): The mountains and seas that constitute the world, according to ancient Indian cosmology. The nine mountains are Mount Sumeru at the center of the world and eight concentric mountain ranges that surround it. Eight concentric seas separate these mountain ranges. According to The Dharma Analysis Treasury, the eight circular mountain ranges are, from the innermost out, Yugamdhara, Īshādhāra, Khadiraka, Sudarshana, Ashvakarna, Vinataka, Nimimdhara, and Chakravāda-parvata. All these mountain ranges are made of gold except the outermost, Chakravāda-parvata, also known as the Iron Encircling Mountains, which is made of iron. Mount Sumeru and the eight mountain ranges are each separated from one another by a sea. The distance between Mount Sumeru and Mount Yugamdhara is eighty thousand yojanas. The inner seven seas are of fresh water, while the outermost sea, just inside the iron mountain range, is salty. In this sea are four continents—Pūrvavideha in the east, Jambudvīpa in the south, Aparagodānīya in the west, and Uttarakuru in the north. See also four continents.