If we are talking about that area around an island the Japanese claim but the Chinese reject their claim to that island, then I think we need more proof it is really there.The story I remember came after the 2010 incident when China cut off rare earths to Japan after the arrest of a Chinese fishing captain. He was released.
Later, 1 or 3 years later, the Japan publicize this find.
Since then, nothing has happened.
They can make the excuses of cost of underwater mining or whatever, but until they dig it up and refine it, you do not know what you have.
That is mining, it is a high risk business. If it is really there, they better have all 17 elements. The heavy elements are particularly hard to find, and China supplies most of that to everyone.
So if China cuts off the rare earths to USA and Japan, they will take at least a year to get the ore and get the elements out, then they have to hope and pray that is the real stuff they need.
At this point, although the future could prove that Japan claim is true, right now that is totally wishful thinking IMHO.
But if they go ahead with it, good luck to them. They will not have to buy from China anymore.