The treasure is dropped by one of the Gatling Groinks on sublevel 7 of the Subterranean Complex in the Valley of Repose upon its defeat. There are two of these enemies on that level, but the one that carries it is always the one atop a tower. Attack that one from behind, preferably with Purple Pikmin. It is recommended to clear the area of bomb-rocks before venturing out with Pikmin, as they can be easily set off in the fight.
Note that because of the way sublevel generation works, it is possible for the Gatling Groink to not spawn, and as such, for the player to be unable to obtain this treasure unless they reset and try a new sublevel layout, as detailed here and here.
“This appears to be some kind of hyper-advanced, light-refracting computer component. Perhaps I can use it to upgrade the ship with an imagination.”
“This will solve any problem, given enough time! This CPU's true processing grit sells itself.”
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