Lake Mashu, Hokkaido | That Blue Looks Like It's Been Edited — It Hasn't
I sent a photo to a friend and he asked if I'd retouched it. I hadn't. That's just the color. Lake Mashu's blue is deep enough to look unreal — and being able to see it clearly at all is a matter of luck.
🚩 NTJ Firsthand Travel Story
When I sent the photo to a friend, he asked if I’d edited it.
I hadn’t. That’s just the colour.
Lake Mashu’s blue even has its own name — “Mashu Blue.” It’s a caldera lake formed after a volcanic eruption, with no rivers flowing in or out, so the water is almost untouched by the outside world; its transparency has been measured as the highest in the world. When sunlight enters, the blue isn’t the sky reflected — it’s a blue the lake itself holds.
Deep, still, a blue that leaves you speechless.
Seeing it at all is a matter of luck.
Lake Mashu has another name — “Foggy Lake Mashu.” For more than half the year it’s covered in thick fog, and many people come all this way and leave without seeing a thing. I went in winter, under a cloudless sky, with the lake and the snowy peaks laid out clearly in front of me.
In the middle of the lake is a small island called Kamuishu, which means “island of the gods” in the local Ainu language. No one is allowed to set foot on it — it just floats there in the centre of the lake, so quiet it makes the whole scene feel even more unreal.
The Story of This Place
Start with what the record shows. Lake Mashū is a caldera lake: volcanic activity in this area began roughly 30,000 years ago, and a huge eruption about 7,000 years ago collapsed the ground into the caldera you see today — one of the youngest caldera lakes in Japan (nested inside the far larger Kussharo caldera). The little island at its center, Kamuishu, is a lava dome: around 4,000 years ago a fresh eruption on the eastern side raised Mount Kamuinupuri while a lava dome piled up at the caldera's center, and only its crown breaks the surface. Kamuinupuri kept erupting until about 1,000 years ago, when a final blast tore off its summit — the last volcanic act here. The lake runs about 20 km around and some 211 m deep, ringed by caldera walls 150–350 m high, and, strangely, not a single river flows in or out, yet its water level barely changes. It once held a world record, too: in August 1930 its transparency was measured at 41.6 m, beating Lake Baikal — though that is history now, as an August 2015 reading found just 17.2 m, a decline officials list as cause unknown (one theory ties it to the 1952 Tokachi-oki earthquake). In 2001 it was named a Hokkaido Heritage site, and today it sits within the special protection zone of Akan-Mashu National Park. That famous aura of the \"misty Mashū,\" by the way, only sank into the national imagination after singer Akira Fuse's 1966 hit song of the same name.
Now the legends — and these belong in a separate column from the facts. It is said that the Ainu long called this place \"Kamuito,\" the Lake of the Gods, and held it sacred, with neighboring Mount Kamuinupuri revered as the mountain where a god dwells. Ainu oral literature also passes down a story: as the tale goes, two villages in the Sōya region went to war, and a grandmother from the defeated side fled with her grandchild, only to lose him along the way. Alone at the lakeshore, she begged the sacred mountain for a place to rest, but grief for her grandchild left her unable to move, and at last she turned into the island at the lake's heart; her tears, legend says, became this land's rain, mist, and blowing snow. That is why \"Kamuishu\" really means \"god\" (kamuy) plus \"old woman\" (shu) — the Island of the Grandmother Who Became a God, not simply \"island of the gods.\" As for the sayings that \"if you see Mashū clear of mist, your wedding will be delayed\" or that \"drinking the lake water delays marriage,\" honesty demands a note: those are not ancient Ainu legend but modern urban superstitions, spread — like everything else — on the coattails of that 1966 song (the water one, some say, was invented to stop tourists carrying the lake home in bottles).
Knowing all this changes what you feel at the overlook. That water in front of you — blue-black, utterly still, taking nothing in and giving nothing out — is a volcanic wound, a lake that once ranked clearest in the world, and an island that was once a grandmother. No wonder that when the mist rolls in, the whole lake looks like it is weeping for someone.
Choosing among the three observation decks
Lake Mashu has three observation decks, each with a different angle. The First Observation Deck is the main one, with the largest car park and the widest view, and you can see Kamuishu Island. The Ura-Mashu (back) deck is on the other side of the lake, with less fog than the first and a slightly different view of the water, but it’s a longer drive. The Third deck’s view is somewhat blocked by trees and is generally skipped. On a self-drive trip, prioritise the First deck and swing by Ura-Mashu if you have time to spare.
Foggy Lake Mashu: when’s the best time to go?
Spring (April–June) and summer (July–August) are the foggiest seasons — which is where the name “Foggy Lake Mashu” comes from. Autumn and winter (October–March) have a higher share of clear days and better odds of seeing Mashu Blue, but winter roads are icy and require snow tyres. There’s no way to guarantee a clear view; just check the day’s forecast and decide.
Planning the surrounding route
From the First Observation Deck you can link up several spots within a 30-minute drive: Mt. Iō (Atosanupuri) — the steaming vents and sulphur streaks of an active volcano; Sunayu on Lake Kussharo — whooper swans in winter; and Teshikaga town — for a bowl of Mashu ramen (the local specialty). Strung into one self-drive day, these four cover about 180 km — a full itinerary.
Getting there and costs
About an hour by car from Kushiro, about 1.5 hours from Obihiro. Parking at the First Observation Deck is about ¥480 (summer peak) and free in winter. There’s no direct public transport — you’ll need to drive. Self-drive itineraries and car rental in Hokkaido can be booked via Klook or KKday.