What are the 9 mountains and 8 seas? 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

One Piece Wiki:


  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.