Hokkaido: Hiking Japan's mysterious Lake Mashu

Laying eyes on Lake Mashu, often considered Japan's most beautiful lake on the northerly island of Hokkaido, is not good luck. Usually enveloped in dense fog, legend has it that those who see Lake Mashu's surface beneath the mist will either a) not be able to marry until later in life, b) break up with their partner if they're in a relationship, or c) be unable to get ahead in their career.

You can imagine my concern, then, at finding the crescent-shaped lake in full view when my Walk Japan tour group and I arrive for our morning snowshoe hike around its circumference. Then again, I tell myself, the legend was probably created to appease travellers distraught by the fact that they weren't able to see the lake at all. Because, ooph, it really is something.

Created when snowmelt and rainwater pooled in the caldera of an active volcano that erupted 7000 years ago, Lake Mashu is surrounded by steep, forested cliffs, and beyond them the peaks of other volcanic mountains found here in Akan National Park. Under today's dove-grey skies, it looks like something from a Hokusai print.

As we strap on our snowshoes and start scuffing through the powdery snow, our guide Takuya tells us Lake Mashu's tagline is "The Lake of Mystery", and not only because of its usual fog shroud. "Its water level never changes, even though there's no water flowing in or out," he says as we gaze down into its depths. He points out a little island named Kamuishi in the middle of the lake, which he says is actually the peak of a volcano rising to the surface through the 211-metre-deep waters. "Hokkaido's native Ainu people call that the 'Island of the Gods'," says Takuya, "probably because humans can't visit it." The lake is completely inaccessible, he says, surrounded as it is by these sheer 400-metre cliffs.

This lack of human interaction with its waters is just one of the reasons why Mashu is one of the clearest lakes in the world, second only to Siberia's Lake Baikal. Because it's not fed or drained by any river there's a lack of plankton or sediment dulling it, plus its volcanic rock walls act as a natural filtration system. It's this clarity that gives Lake Mashu its usual brilliant blue colour, says Takuya, a shade so unique it needed its own name: Mashu Blue. Just imagine how this place would look in spring, he says, the surrounding cliffs upholstered with lush green grasses and wildflowers, the lake its most vibrant shade. And we really do need our imaginations, since today's cloud-capped sky has turned it a murky shade of grey. Still, there's no denying its magnetism.

We continue hiking through a grove of birch trees, their bare silvery branches reaching towards the grey sky, and spot sika deer, the largest deer in Japan, grazing in the distance. Takuya points out rabbit droppings and amber-coloured shoots emerging from the tops of the snow-draped birch trees, more evidence of the life miraculously surviving in this unforgiving environment.

Soon, having hiked just a small portion of this mysterious lake's 20-kilometre circumference, we stop for a snack of fresh mochi cakes. It's minus 10 degrees and we're all complaining of the cold. Until we notice the "diamond dust", that is – tiny ice crystals blowing through the air and reflecting the sun's weak rays – only seen in such inhospitable environments. We watch it glitter over this pristine lake. And I think, just for a moment, that it could almost be worth the collapse of my marriage or career to have seen it.