Zhang Shengwu, a villager in China's eastern Anhui province, recently launched his five-ton sub into the river near his rural home, state broadcaster CCTV reported Thursday.
Footage from CCTV showed Zhang piloting the seven-metre (23-foot) steel craft from its hatch while surfaced and diving down with the hatch closed.
After seeing footage of submarine construction on TV in 2014, Zhang, a former carpenter who also worked in shipping, decided to start building his own.
"I've been around the water for many years and seen iron boats and wooden boats, but I had never seen a boat that can dive into the water," Zhang said.
"I thought, if others can do it, I can do it too."
Despite his wife's opposition to the "expensive, risky, useless" idea, Zhang began pursuing his submarine dream, first by spending 5,000 yuan ($700) on steel plates, engines and other materials.
The inventor launched his "first generation" sub in 2016, but it leaked.
"It was like a dream. I was both afraid of it leaking and hoped to go deeper," Zhang said.
Years later, after spending another 40,000 yuan on a new hulking steel structure, pouring two tons of concrete into the bottom of the submarine, and adding two ballast tanks, Zhang's Big Black Fish was ready for sea trials.
Unlike the Chinese navy's advanced nuclear-powered submarines that can spend months submerged, Zhang's sub uses a small battery and electric motor, can travel just four knots per hour and needs to surface after half an hour.
He plans to build an even bigger submarine in the future.
Zhang is not the first Chinese inventor to take a deep dive into submarine construction.
In 2015, a villager in China's northern Shaanxi province accumulated a debt of 200,000 yuan to fund the construction of his 9.2-metre-long submarine.
In 2009, karaoke bar worker Tao Xiangli cruised around a local reservoir in Beijing in a homemade submarine.
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