Underworld
You go outside to bask your forehead in the sun, but then you see these creatures with lids for heads and fins for feet. Uh… where did they come from? From the underworld. Nothing like them exists in this world. They obviously came from the underworld.
めい, みょう
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These underworld creatures must be out and about because it's May (めい), and the weather is so fantastic and sunny right now. They don't get to see the sun down there in the underworld, so every May they turn up in droves to experience it.