Lake Motosu-ko does not freeze.
It sits at around 900 metres on the northwestern flank of Mount Fuji, the deepest of the five lakes at approximately 121 metres. Even in winter the surface temperature stays just above four degrees Celsius. The depth is the reason. The water does not mix easily, and the deeper layers hold enough heat to keep the surface from freezing. The other four lakes freeze. This one does not.
The lake exists because of lava. At some point in the distant past, flows from Fuji moved across this landscape and blocked what was then a much larger lake that dominated the area north of the mountain. That lake was divided into three. Motosu-ko, Sai-ko, and Shoji-ko. The lava that separated them eventually cooled and solidified, and over subsequent centuries the dense forest of Aokigahara grew across it, covering the old flow field in a layer of trees so thick that the rock beneath is barely visible from above. The forest looks ancient and self-contained. It is neither. It is growing on the frozen evidence of an eruption.
The three lakes look separate now. But they are believed to remain connected underground through porous volcanic rock, groundwater moving slowly through the same material that divided them in the first place. It is why all three sit at exactly the same surface elevation of 900 metres. Change the water level in one and, eventually, you affect the others.
Motosu-ko is the westernmost and least developed of the five. Its shores are barely built around, a handful of campgrounds, the water open and quiet in a way the more visited lakes are not. But it has been recording Mount Fuji for a long time. Researchers have drilled sediment cores from the lake floor, recovering an 8,000-year record of volcanic deposits that has helped refine the eruption chronology of the mountain itself. Each layer of scoria in those cores is tied to an eruption, some of them not clearly visible in the land-based geological record. The lake floor preserves what the landscape above it does not always show.
The view from the northern shore is the one printed on the 1,000-yen note, based on a photograph taken by Koyo Okada in 1935. Fuji reflected in still water, the mountain inverted on the surface of the lake. That stillness comes from the depth, the structure of the water column, and the same volcanic history that created the lake and keeps it from freezing and connects it underground to its neighbours.
The lake is a product of Fuji. It has also been one of the better witnesses to what Fuji has done.
Copyright © Tales from the Horizon, 2026
