Japan found hundreds of years’ worth of the minerals used in batteries and EVs

Fun fact most, of the production came from the Mountain Pass rare earth mine, it's on the right side of the highway when you drive from Vegas to LA

If it'd be pure economics, then yes. But Japan and China are not necessary the best friends, so there is a strong geopolitical factor at play. China has the (quasi) monopoly and China with strategic embargo/tariffs/creative strings attached could easily grind the tech manufacturing in Japan to halt, impacting it's economy severely. This is not theoretical, they already impacted Japanese economics this way.

So now Japanese politics has a strong incentive to change the situation, creating a second source. It might be more expensive as long as the supply lines are still open, but it's a strong hedge against politically motivated supply line disruptions.

Simply said, exploiting these deposits even if they are more expensive than the market price can be seen as an insurance. It also strengthens Japan's negotiating position when their industry can't be held hostage so easily.

ps.: Also keep in mind, that it takes considerable time (years) to develop a mining and refining operation, so they can't just wait until a disruption occurs before starting with the mining.

> They're just very polluting to mine

Why exactly is that? I can't see why some polluting process would be common to all of them, and unavoidable.

My limited understanding is the toxic part is less the mining and more the refining — the acid baths that raw material goes into to extract the metals. I’d assume that’d need to be on land.

Mining in general is problematic. If you break up a bunch of rock and leave it lying around, all kind of stuff will leak from it.