Lake Toya formed 110,000 years ago when a massive eruption collapsed the earth into a nine-kilometer-wide caldera, creating the eleventh-largest lake in Japan and one of only two in the country that never freezes. The volcanic forces that shaped the basin remain active beneath its surface — Mount Usu, standing 733 meters on the southern shore, has erupted four times since 1900, most recently in 2000, when evacuations emptied the town of Toya and reshaped the landscape overnight. Today the caldera lies at the center of Shikotsu-Toya National Park, a 993-square-kilometer reserve designated in 1949 to protect the region's interconnected volcanic systems, hot spring networks, and alpine forests. Noboribetsu, fifteen kilometers southeast, serves as the primary gateway to the caldera. The town rose to prominence in the 1850s when sulfur miners discovered Jigokudani — Hell Valley — a barren volcanic rift where steam vents and sulfur streams still pour from fissures in the bedrock. By the early twentieth century, mineral-rich onsen had made Noboribetsu one of Hokkaido's most visited resort towns, and its rail connection to Sapporo ensured steady access for travelers moving deeper into the park. The road linking Noboribetsu to Lake Toya opened in 1932, cutting travel time between the two centers to under forty minutes and establishing the caldera as a natural pairing for visitors already drawn to the valley's hot springs. Lake Toya tours from Noboribetsu typically combine caldera sightseeing with volcanic terrain exploration. Departure points cluster near Noboribetsu Station and the onsen district, with routes tracing Route 453 northwest through forested ridgelines before descending to the lakeshore at Toyako Onsen, the basin's primary settlement. From the waterfront, cruises depart hourly to Nakajima — a forested island cluster in the lake's center — where trails wind through Yezo spruce and offer unobstructed views of Mount Yotei to the northwest. The southern shore climbs steeply toward Showa-Shinzan, a 398-meter lava dome that erupted between 1943 and 1945, rising from farmland over the course of eighteen months. Today a paved trail leads to an overlook where visitors can see the dome's fractured surface and the steam still escaping from its flanks, evidence of the magma chamber three kilometers below. Guided tours departing from Noboribetsu allow travelers to move between both volcanic systems in a single day, pairing the sulfur streams of Jigokudani with the caldera's open water and island forest without navigating unfamiliar rural bus schedules or managing multiple ticket purchases.