In a narrow valley in Chichibu, spring water seeps out of a rock face all winter. When it is cold enough for long enough, it freezes on the way down and builds into a curtain of ice: around thirty metres wide and eight metres high, growing all season.

It is the only naturally formed ice formation of its size in Saitama. The other famous Chichibu icicles are helped along by sprayed water. These are not.

They light it after sunset on selected days only, usually starting about a week after the season opens. The colour changes slowly from white to blue to green, and the stream below it picks up the reflection. The light-up runs from sunset to about seven in the evening.

We start in the early afternoon, so you keep your morning. Daylight is spent on Chichibu itself, which is a worthwhile place in winter and empty at that time of year: Mitsumine-guchi valley, the Arakawa river gorge where the rock is folded and twisted into shapes that make the geology obvious, and Chichibu town with its old shop fronts, miso and sweet-potato shochu.

We reach the icicles before sunset so you see them in daylight first - which matters, because you cannot judge the scale once it is dark and coloured - and stay through the light-up on the nights it runs.

Then a hot spring on the way home, which after two hours standing in a frozen valley is not optional as far as we are concerned.

Your private van, driver-guide and the whole day are yours alone. No other customers join.

In a warm winter the ice does not form properly and the site does not open. We check before your date and will tell you honestly - you can move your date to another day. We will tell you before the tour.