Soranuma-dake (空沼岳) is a peak sitting between the hot springs at Jozankei Onsen (定山渓) and the lake Shikotsu-ko (支笏湖). As this area gets a huge amount of snow in the winter, there are a number of marshes that dot Soranuma-dake's slopes, namely Bankei-numa (万計沼), Mamisu-numa (真簾沼), and the eponymous Sora-numa (空沼). The etymology of the mountain's name is unclear, but it may come from one of two sources: 1) from the old name for the nearby river Izari-gawa (漁川), which the Ainu called raruma-nai (as in (so)raruma, which would have drifted to 'soranuma' over the years), or 2) from a dried-up stream on the mountainside, which would have been called 'Kara-numa' (空沼, lit. 'empty marsh’)—this uses the same Japanese characters as 'Sora-numa' would, and over time the reading may have changed over. At Bankei-numa, there are two huts: Bankei-sansou (万計山荘) and Soranuma-koya (空沼小屋)—these have long been popular with adventuresome folks on the mountain, even in winter. You can also make the traverse from Soranuma-dake to Sapporo-dake if you're in the mood for a longer walk.
You'll find the trailhead at the end of a forest road which follows the Bankei-sawa (万計沢) stream from the Soranuma Tozan-guchi bus stop (空沼登山口バス停). You'll cross a bridge and climb up into a mixed forest of evergreens and broadleaves; but it's the huge Ezo spruce around here that will really catch your eye. With a forest like this it's little wonder that Sapporo is known for its sprawling nature.
From the right bank of the stream you'll shortly cross to the left bank. A little ways after this, you'll leave the side of the stream for a bit; when you meet back up with it you'll have arrived at the small pond called Bankei-numa. In the trees by the shore of the pond lie big Bankei-sansou lodge and the smaller Soranuma-koya hut. If you can spare the time, taking a couple of days to hike the mountain and staying overnight at Bankei-sansou is a lovely way to spend a weekend.
A little ways above Bankei-numa you'll find the comparatively larger pond called Mamisu-numa. After wandering along the banks of the sprawling pond, the trail will head up a steep slope. You'll emerge on a ridgeline looking down onto the tops of trees. On this ridge you'll run into the junction with the trail from Sapporo-dake (札幌岳) before shortly thereafter reaching the summit of Soranuma-dake.
Though it doesn't have much of the feel of being a major summit, the view from the top is outstanding. In particular, the mountains around Shikotsu-ko, especially Eniwa-dake (恵庭岳), are spectacular to see. Which is not to mention the hills around Jozankei or the massive cone of Yotei-zan (羊蹄山), Hokkaido's Mt. Fuji, in the distance. At times you may even see the head of a chipmunk or two poking out of the surrounding foliage.
If you were to continue along the ridge from the summit, you'd soon see the pond called Sora-numa as well; but there is no trail down that way. Your trip down will follow the same trail you came up.
If you're looking to make the traverse to Sapporo-dake, from the junction you'll head onto an undulating ridgeline, through a forest of Erman's birch and dwarf stone pine. As you descend, you'll likely start seeing sasa bamboo as well.
From the pond of Hyoutan-numa (ヒョウタン沼), looking west, you'll see a slim mountain called Sausu-yama (狭薄山). As you climb the steep eastern flank of Sapporo-dake, you'll merge with the Toyotaki trail (豊滝コース) and shortly thereafter arrive at the summit.
From Soranuma-dake to Sapporo-dake takes about 2 hours. Going the other way, Sapporo-dake to Soranuma-dake, takes about the same.
In winter you'll follow the same trail. You can handle most of it on skis, but keep in mind that the climb up to Sapporo- dake is quite steep. Beware of cornices.
- You can take a Hokkaido Chuo Bus (北海道中央バス) from Makomanai Subway Station (真駒内地下鉄駅) to the Soranuma Tozan-guchi bus stop (空沼登山口バス停). From the bus stop to the trailhead itself is about 1 km.
- The Bankei-sansou (万計山荘) lodge is open year-round and can accommodate 50 people. It's free, although they accept donations to upkeep the place. From June to October, on weekends and holidays, there is a manager stationed there. You can collect water from the pond, but treat it first. Soranuma-koya was closed for repairs as of the writing of this book in 2010; as of 2015, it appears to still be closed.
- Just below the ridge where the trail meets the junction to Sapporo-dake, you can find some water in early summer. By later in the summit it will be mostly dried up.
In May there will still be some snow around the summit, so trek carefully. The leaves come out in June and turn red in October; around the ponds and marshes it's particularly beautiful. Staying at the huts and ski touring across to Sapporo-dake is a popular winter trip.