A massive, 'semi-infinite' trove of rare-earth metals has been found in Japan

I hope "semi-infinite" sees wider adoption in our lexicon.

Clearly you did not study journalism.

I don't know if the exploitation will be fast enough to make a huge difference, at least for the time being. It being so difficult to get to it gives other leading countries, namely China, to re-think their exploitation and sale of these materials. They could find a way to stabilize the supply side market with this extra time. Especially since so much of the manufacturing using these materials is done in their country. Still though, I agree this new supply should change buying and manufacturing behavior.

Is the find actually cheaper to extract than current resources, though? This may act more as a thing to ramp up if supply shocks drive up prices.

Which may enable new technologies that weren't cost effective before.

Or some other technologies will replace the materials used, rendering the new deposits useless, from a cost-based perspective.

By the time Japan is able to extract and use that supply the trade war with China would be long over or one would hope so. The supply is several kilometers down. One would have to build a rig and then find a way to efficiently pump that material back up in large enough quantities for commercial usage (i.e. cheap enough to compete with China's supply)

China owns over half of the mines in Africa. It subsidised the infrastructure to get mined materials back to China.

That is all to say it now has extremely cheap access to any raw material it wants. Japan's super-expensive submarine sludge isn't going to make any difference.

I would argue it probably only needs to last longer than a news cycle.

Indeed. Which in practical, commercial terms is semi-infinite, since commercial mining does not involve a never-ending search for additional potential sources of material, only the mid- to long-term sufficient exploration and development of economically feasible sources. Where mid-term could mean approximately one human generation, and long-term a human lifetime.

Another example of such long-range planning in commercial ventures is the lumber business in North America and Scandinavia (that I know of), deliberately and systematically re-generating forests for continued harvesting while keeping their sight on a 80-100 year horizon.

Don't know enough about the metals involved to answer, but usefulness is often a function of availability. They may be usable, but there may be more readily available alternatives that have been good enough. If this supply makes new materials even more available, it could still change things.

In the last few centuries, all developments of technology and the economy have repeatedly involved "quite a leap". I find it both intriguing and exciting.

As opposed to how they are now?