Kameiwa Cave

A small waterfall drops through a hole cut in rock in a wooded valley in Kimitsu, and for a couple of weeks twice a year the sun rises at exactly the right angle to shine through the opening. The light beam and its reflection in the pool below form a shape that reads as a heart, which is why a quiet forest stream in inland Chiba became, briefly, one of the most photographed places in Japan.

The hole is not natural. It was cut in the Edo period as a tunnel to divert the river and open up new rice paddies — a technique called kawamage that was used across the Boso peninsula — and the waterfall is what happens where the diversion drops. Locally it gets called Nomizo Falls, though strictly that is the fall downstream. The rest of the year the valley is simply a mossy stream with a boardwalk, which is a fine thing to have twenty minutes off the Aqua-Line.

The kawamage technique behind the tunnel was common practice across the Boso peninsula in the Edo period, wherever a river bent sharply enough that farmers judged it worth the labour of cutting through the rock at the neck of the bend rather than following the water's own course. Straightening a river this way did two things at once: it shortened the loop enough to drain a stretch of the old riverbed and turn it into paddy land, and it left the diverted water to carve a new, shorter channel that often produced exactly this kind of drop where the tunnel met open air. Most of these Edo-period diversions across the peninsula are unremarkable ditches today, indistinguishable from any other minor watercourse. Kameiwa's became famous purely by accident of geometry and latitude — nobody who cut the tunnel could have known that its opening would one day align with sunrise at the equinox weeks closely enough to throw a heart-shaped reflection into the pool below, and the effect went essentially unnoticed by outsiders until photographs of it began circulating decades after the fact.