Iwami Tatamigaura

In 1872 the Hamada earthquake lifted a section of sea floor about a metre out of the water, and it has stayed there. The result is roughly 49,000 square metres of flat rock shelf at the foot of a 25-metre cliff, ruled by a grid of natural joints so regular that it looks like tatami matting laid out to the sea — which is exactly what the local name for it, senjojiki, describes. It was designated a national natural monument in 1932.

The rock is Miocene sandstone about sixteen million years old and it is full of things: shellfish, driftwood, and whale bone, visible in the cliff face and the shelf itself. Spherical concretions the size of cannonballs sit on the platform. You reach it through a tunnel driven through the headland, which adds a small amount of theatre to what is otherwise a geology lesson you can walk on. Come near low tide — at high water most of the platform is under the sea.

The name senjojiki, 'a thousand tatami mats,' is not much of an exaggeration once you're standing on the shelf — the joint pattern really does divide the rock into a grid of roughly rectangular panels, and walking across it feels closer to crossing a vast paved courtyard than scrambling over a natural formation. Because the earthquake lifted this section of coastline in a single event rather than through gradual erosion, the platform sits at an unusually uniform height above the current waterline, which is part of why it stays exposed for hours around low tide rather than only briefly. The cliffs above are riddled with sea caves carved by the waves before the uplift, and the whole stretch of coast here — part of the Sanin Kaigan region — has a raw, undeveloped character with none of the guardrails, gift shops, or paved paths that soften more famous Japanese coastal attractions, leaving the geology to speak largely for itself.