Naruto Whirlpools

The Naruto strait is 1.3 kilometres wide between Shikoku and Awaji, and it connects the Inland Sea to the Pacific. The tide reaches the two sides at different times, so a difference in water level of up to about 1.5 metres builds across the strait and the water pours through at speeds around twenty kilometres an hour - among the fastest tidal currents anywhere. Where the fast central flow meets slower water at the edges, vortices form, and on the biggest spring tides they can reach around twenty metres across. Boats run out from Naruto to sit beside them, and Uzu no Michi, a walkway built into the underside of the Onaruto Bridge, has glass floor panels 45 metres above the water directly over the strongest flow. Timing is everything: the whirlpools appear for a window around each tide change and are absent between.

What you actually see

• Vortices up to twenty metres - forming where fast central flow meets slower edges.
• A twenty kilometre an hour current - among the fastest tidal flows in the world.
• Boats alongside - vessels holding position beside the strongest whirlpools.
• The bridge walkway - glass floor panels 45 metres above the water.

When to go

Spring tides in spring and autumn, at the published times. Half a day.

Honest expectations

Whirlpools appear only in a window around tide changes and can be small on neap tides - check the forecast tables before booking. Boats are rough and cancelled in swell. The glass-floor walkway queues. Expectations set by photographs of the largest events are often disappointed.

Nearby

Awaji Island is across the bridge; Tokushima is south.