Lake Suwa, the largest lake in Nagano, freezes over entirely every winter. Some years, a jagged ridge dozens of centimeters high forms in the ice. This stunning natural phenomenon is called omiwatari, or "God's crossing." Omiwatari occurs when the ice surface stretches due to differences in temperature between day and night. Three ridges usually form: two from the southeast part of the lake to the north, and one from the southwest to the east. When viewing omiwatari, be sure to bundle up for the cold and stay out of dangerous areas.
Sadly, due to the effects of climate change, sufficiently cold periods for omiwatari have become much rarer. However, there are many other winter activities at Lake Suwa, a favorite being ice fishing for smelt. There are a number of fishing facilities with poles and other equipment available to rent.
Many more winter attractions line the lakeshore, such as onsen resorts, hot spring-fed footbaths, scenic shrines and temples, famous local dining, and the restored Takashima Castle.
Take the JR Azusa limited express train from Tokyo's Shinjuku Station to Kami-Suwa Station. The trip takes about 2 hours 15 minutes.