Yonaguni Monument

Boats leave Kubura and reach the site in about 20 minutes, and they only go when the formation sits on the leeward side of a north wind and the current has eased — the area is notorious for its currents, so a booked day can be called off at the harbour. Descend onto the platform and the scale registers first: a single mass roughly 100 metres long and 50 wide, standing up to 25 metres tall, with the shallowest steps at around 5 metres, the ruins running down to 20 and the foot of the formation at 26.

Orientation runs on names the original discoverers and researchers gave the features. They may not be scientifically correct, but no better names exist and they work underwater. The Main Terrace and its stairs form the core of the circuit; the Loop Road, a ledge 5 metres wide, encircles the base of the formation on three sides and gives you a line to follow when the flow picks up. The Twin Megaliths, Water Trench, Triangle Pool and Turtle Monument sit off that route.

The Arch Gate is the one squeeze on the dive, a passage 1 metre wide and 2 metres high that reads like a short tunnel; take it one at a time with nothing trailing. Work back up the terraces to finish at around 5 metres, where the light shows the right-angled edges best — and where the water is busiest, since the island lies in open sea with no protective reef to break the swell before it reaches the shallow steps.