Yonaguni Monument

The Monument is a mass of layered sandstone the size of a building, cut into flat terraces, straight edges, and stark right angles, found in 1986 by local diver Kihachiro Aratake while scouting hammerhead grounds. Whether people shaped it is a genuine, unresolved argument. Marine geologist Masaaki Kimura spent years cataloguing what he interpreted as tool marks, drainage channels, and even carvings; most geologists respond that this sandstone naturally fractures along horizontal bedding and vertical joints, producing staircase forms without any hands involved, and that similar geometry appears on the island's shoreline. Diving it, you sweep across the main terrace, duck the surge behind the prow-like corner, and understand why both camps sound so sure of themselves.

その瞬間のために: The terraces and right angles themselves, the natural-versus-carved debate, big Kuroshio visibility

地形: Massive stepped sandstone formation rising from about 26 m toward the surface

流れ（カレント）: Kuroshio-driven flow and surge over the terraces are routine; calm days are the exception to plan for.

エントリー: Short boat runs from the island's harbors; entries are timed around surge and the current wrapping the south coast.

危険生物・リスク: Surge over the terraces, current at the corners, and swell surface intervals; listen hard to the local briefing.

ナイトロックス: ここでは効果的です——このエリアのプロファイルは、ナイトロックスが無減圧潜水時間を有意に延ばせる水深帯にあります（適切なトレーニングを受けていることが前提です）。