Minamizawa Drainage Tunnel Site

How was it possible to dig such an underground tunnel like this one in the Edo period?

The Minamizawa Drainage Tunnel was built to drain water from the mining tunnels of the Aikawa Gold and Silver Mine. It was dug by hand, using only hammers and chisels, over the course of five years starting in 1691. The tunnel is roughly one kilometer long. Once the sections were successfully joined, the water immediately began to flow out, clearing many of the mine tunnels that had previously been flooded. The tunnel was built using Western surveying techniques. They used a revolutionary method of digging from six locations simultaneously, yet the error between the mountain and sea openings was less than one meter. The level of technical skill demonstrated here was the highest in the world at the time!

The tunnels are shaped like a pentagon, just like a piece of Shogi!

The cross-section of the tunnel resembles a shogi piece, which is shaped like a pentagon. There are several theories about this. Some believe it is to make the tunnel stronger, while others think it helps air circulate and water drain better. Still others believe it is to show off the workers' skills. It is unclear which, if any, of these theories is correct. The hard rock made it difficult to dig more than 16 meters of tunnel each month. It's interesting to see how the skill of the workers shaped the tunnel differently, with some parts shaped like a perfect pentagon and others rough.