From https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q13148895433 it seems 아레 means the below/beneath senses of under and 밑에 means under some object or at the bottom. You'd say at the bottom of the sea, say, not below/beneath it. Similarly you'd say below/beneath the sky, which is not an object either, not at the bottom of the sky unless maybe you're off the ground . . .