The word onbashira means "honored pillars," and the festival's purpose is deceptively simple: replace the four massive wooden posts standing at each corner of the shrine complex's four buildings. That means sixteen fir trees must be felled, transported down the mountains, and raised into position. The largest pillar, the ichino-hashira or "first pillar," stands approximately 16.6 meters tall. The logs for the Upper Shrine come from momi fir trees on Mount Okoya in the Southern Yatsugatake Mountains, while those of the Lower Shrine are sourced from the Higashimata forest in the town of Shimosuwa. Each log is dressed in red and white regalia, the traditional colors of Shinto ceremony, and fitted with ropes for the teams of hundreds who will haul them to the shrine. The festival unfolds in two major phases: Yamadashi in April, when the logs are brought down from the mountains, and Satobiki in May, when they are paraded to their final positions and raised at the shrine's corners.

The festival's most heart-stopping moment is the kiotoshi, or "tree falling," when the massive logs are slid and dropped down steep hillsides. Young men clamber onto the trunks, gripping ropes and each other, as the wood lurches forward and accelerates downhill. The Lower Shrine's kiotoshi is the iconic version, originating in the Meiji period after 1895, when the Higashimata forest became the designated source and the current route through its infamous hillside was established. The danger is not theater. In 1992, two men drowned when a log was dragged across a river. In 2010, two men, Noritoshi Masuzawa, 45, and Kazuya Hirata, 33, died when a guide wire snapped as a pillar was being raised at the Suwa Grand Shrine. In 2016, another man fell to his death during the raising ceremony. The festival's reputation as the most dangerous in Japan is earned with each iteration.

The Onbashira Festival is popularly reckoned to stretch back over 1,200 years, rooted in the veneration of Suwa Daimyojin, the deity of wind and water worshipped at Suwa Grand Shrine since antiquity. Samurai clans revered him as a patron of hunting and warfare. Unlike most Shinto shrines, the Upper Shrine complex has no honden, the building that normally houses the deity. Instead, its objects of worship were the sacred mountain behind the shrine and, remarkably, the shrine's high priest himself, the Ohori, considered to be the physical incarnation of the god. During the upheavals of the Sengoku period, the warlord Takeda Shingen saved the shrine's ceremonies from oblivion. After conquering all of Shinano Province by 1565, he ordered the reinstitution of the Onbashira rites. In 1582, the eldest son of Oda Nobunaga burned the Upper Shrine to the ground, yet it was rebuilt on schedule just two years later, in the Year of the Yang Wood Monkey, as tradition demanded.

The Onbashira Festival of 1944 was unlike any before it. With Japan's wartime conscription draining the countryside of able-bodied men, the enormous logs were hauled and raised primarily by women and elderly men. During that festival, the mayor of Shimosuwa, Tokichi Takagi, was killed in an accident during the proceedings. A monument to his memory now stands at the site, and it has become customary to pray for safety before it. The festival of 1950, the first held after the war, marked a turning point: women were officially allowed to participate for the first time. The ritual's roots run deeper than any one generation. The timing itself carries cosmological significance, falling in the years of the Tiger and Monkey in the Chinese zodiac, corresponding to the theory of the five elements, where wood begets fire, fire begets earth, and the cycle of creation turns. Every six years, the pillars come down, the trees come down the mountain, and the shrine is made new again.

From the air, Lake Suwa sits in its valley like a dark mirror surrounded by the peaks of central Nagano. The four shrines of the Suwa Grand Shrine complex bracket the lake: the Upper Shrine's Honmiya and Maemiya on the southeastern shore near the cities of Suwa and Chino, and the Lower Shrine's Harumiya and Akimiya in the town of Shimosuwa on the northern side. The origins of the onbashira pillars themselves remain debated. Some scholars see them as relics of larger structures, others as boundary markers akin to Korean jangseung. A compelling theory traces the practice to prehistoric tree worship from the Jomon period, thousands of years before recorded history. Whatever their origin, the sixteen pillars standing at the shrine's corners are a statement repeated every six years: that the most sacred things must be continually remade, and that remaking them demands everything a community has to give.