Lake Toya and Lake Shikotsu: Two Caldera Lakes from Sapporo

Beside Lake Toya stands Showa Shinzan, and its story is the reason to come. Between 1943 and 1945 a wheat field began to bulge. Over two years the ground pushed upward and a lava dome rose several hundred metres out of the farmland.

Japan was at war and the authorities suppressed news of it. The local postmaster, Masao Mimatsu, recorded the growth day by day from his window, plotting the changing skyline on paper. His record became the most complete documentation of a volcanic dome forming anywhere, and it is known in volcanology as the Mimatsu diagram.

He later bought the land to protect it from souvenir hunters. The mountain is still privately owned by his descendants — the only privately owned volcano in Japan — and it is still steaming.

Toya is a caldera lake with a group of islands at its centre and Mount Usu, an active volcano that has erupted several times in the last century, on its southern rim. A ropeway climbs Usu for the view over the lake and down onto Showa Shinzan.

The area is a UNESCO Global Geopark, and the most interesting walk here is not scenic but forensic: preserved ruins from the 2000 eruption — a buried road, a flattened bathhouse, apartment blocks with mudflow through them — left in place as an open-air record.

The lakeside town is an onsen resort with hotels along the water, and there is a nightly fireworks display over the lake through the summer season, launched from boats. It pairs naturally with Noboribetsu, which is close by.

Lake Shikotsu is only about an hour from Sapporo, which makes it the easier half-day, and it is a different kind of place: deep, extremely clear, and ringed by forest and mountains rather than by a resort strip.

It is one of Japan's clearest bodies of water, and because it is very deep it does not freeze over even in a Hokkaido winter — the water stays liquid while everything around it is under snow.

In summer the draw is clear-bottomed kayaks and boats over the visible lake floor. In winter the town runs an ice festival where lake water is sprayed over frames and freezes into blue translucent sculptures, lit at night — smaller and stranger than Sapporo's snow festival, and far less crowded.

Onsen sit along the shore, and it is close enough to Sapporo to do as an afternoon rather than a full day.

If you have one half-day, take Shikotsu — it is an hour away, the water is the point, and you can be back in Sapporo for dinner.

If you have a full day, take Toya, and combine it with Noboribetsu's hot springs, which is how nearly every tour structures it. The volcanic story is stronger and the geopark material rewards the longer trip.

If you have a night, stay at Toya. The lakeside onsen hotels with the fireworks on the water in summer, or the snow in winter, are the version of this that people remember.

Both are within Shikotsu-Toya National Park, and if you are also doing Noboribetsu you are moving through one continuous volcanic system all day — which is the actual through-line of a Hokkaido trip and rarely presented that way.