On September 1, 1923, a massive earthquake struck the eastern part of Japan, devastating Tokyo and taking the lives of nearly 150,000. Unexpectedly, one of its aftershocks, called the Tanzawa earthquake, led to a stunning archaeological discovery in January of the following year.

Out of nowhere, in the aftermath of the Tanzawa earthquake, seven wooden pillars appeared in the liquefied paddy fields by the Koide River. Archaeologists were called in, and three more pillars were subsequently found buried in the water.

Made of hinoki cypress, the pillars had a diameter of roughly 25 inches and stood at 11.5 feet tall. It was soon speculated that they must have been the remains of the bridge mentioned in Azuma Kagami, a medieval chronicle of the Kamakura period.

According to the chronicle, the bridge was one of the largest of the time, built over the Sagami River in 1198 by samurai lord Inage Shigenari in memorial of his late wife. Local legend claims that Minamoto no Yoritomo, the ruler of the Kamakura shogunate, died right after the inauguration ceremony of the bridge, having fallen off his horse which was startled by the ghosts of the rival Heike clan. It is said that this gave the locality of Banyu ("horse going in") its name.

The bridge was lost at some point in history and replaced by a ferry by the Edo period. Not long after its discovery, in 1926, its ruins were designated a National Historic Site. Dendrography dated the pillars to between 1126 and 1260, seeming to confirm the theory that they once supported Shigenari's bridge. Interestingly, the Sagami River runs about 1.5 miles away from the site, suggesting that the water changed its course over the past seven centuries.

For decades, the pillars stood as is in the pool, but the exposed parts started to corrode over the years. Today, exact replicas stand in their place on the site, with the original preserved in concrete underground, protected but not to be seen.