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The Arctic Circle is the region around the North Pole.
Literal
Arctic-Circle [topic-とは] North-Pole surrounding [of-の] region [of-の] thing is.
とは here is the definitional/topical とは — '[X]とは: [X] is, meaning X.' Used in encyclopedic or definitional statements. のことである is the formal equivalent of のことだ — 'is (the matter of) X.' ある ('to exist') is the formal copula-adjacent auxiliary used in written Japanese for definitions.