The Yatsuba Dam in Gunma Prefecture

This video shows concrete being poured at the Yotsuba Damn in Gunma Prefecture.

The Yatsuba Dam was originally planned in 1952 to protect Tokyo and the Tone River basin from floods caused by typhoons in the Agatsuma River, one of the major tributaries of the Tone River. However the project was abandoned due to water quality problems, lack of construction technology at the time, and anti-dam movements.
After a long period of time to address these issues, construction of the dam began in 2015, and it was completed on March 31, 2020.
The dam lake was named the "Yanba-Atsuma Lake" out of around 1,000 candidates from across the country.

After years and years of waiting, the dam was finally completed.
There's a roadside station and various tourist facilities around the Yatsuba Dam, so if you're in the area, be sure to stop by.