The Akagi mountain scenery at the beginning of autumn, in Gunma 21 Friday Sep 2012 Original size at 529 × 353 ≈ Leave a comment Ono (Big pond) is a formal name, Oonuma is commonly known. Mt. Akagi’s caldera, 88ha wide, altitude 1345m, depth 16.5m. There is a Akagi shrine in the island.