After Ukizane Soga nearly destroyed the Mikaboshi, their king used Datenseki to build an enormous spherical habitat deep under the ocean, crafted from immense branches. The clan and the people who went with them adapted their bodies to the unstable mineral. That is why raw Datenseki does not pop them the way it pops a mainland user. Shokoku rising is that habitat deciding the wait is over.
Not a second Irishima
Irishima is the vein Japan already found. The habitat is the thousand-year answer to a Soga purge. Hasumi’s later lab notes that vitality alone should not let a body survive the ore’s crush; the printed research still treats the adaptation as the mystery. Ariu’s Datenseki-hardened flesh is the prince version of the same homework. See Ukizane, Shokoku, Ariu.