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Chinese Farmer Builds Handmade Submarine Big Black Fish, a 5-Ton Submersible Capable of Diving 8 Meters with Two Passengers in the Fengle River in Anhui.
On July 2, 2025, residents of Hanshan County in the Chinese province of Anhui witnessed an unusual scene in the waters of a local river. A dark steel structure slid slowly across the surface, emitting a low mechanical noise, and a few seconds later disappeared under the water. It was not a military test nor an industrial prototype. Inside the vessel was Zhang Shengwu, a 60-year-old farmer who built by hand a handmade submarine called Big Black Fish after ten years of night work.
Without an engineering degree, without a formal technical plan, and without family support for much of the project, Zhang decided to develop a functional submersible using steel plates, an electric motor, and common batteries. The result was a small submarine weighing 5 tons capable of diving up to 8 meters deep with two passengers on board. On the same day of the first public dive, the test video was published on Douyin, the Chinese version of TikTok, and quickly spread through the country’s social media. The vehicle’s name caught immediate attention: Big Black Fish.
Zhang Shengwu’s Childhood and the Relationship with the Rivers of the Yangtze Basin
Zhang Shengwu grew up in the Yangtze River basin, one of the most important watersheds in China. Anhui Province, in the eastern part of the country, has a large number of rivers, channels, and water transport, especially in rural areas.
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Throughout his life, Zhang worked in various construction and transportation-related activities. He was a carpenter, welder, and worker in the river navigation sector. Over twenty years ago, he built a small sand and gravel dock in the village of Zhangdu, near the city of Ma’anshan, where he began to observe cargo vessels that sailed through the region on a daily basis.
In an interview with state broadcaster CCTV, Zhang reported that this constant interaction with boats sparked his curiosity.
He stated that for years he saw wooden boats and steel ships crossing the river, but he had never witnessed a vessel capable of diving. This curiosity would ultimately become the starting point for the submarine project.
The Idea of Building a Submarine Came After a Television Program
The direct inspiration for the project came in 2014, when Zhang watched a television program showing a person building an experimental submarine. According to him, the segment immediately sparked a simple question: if someone else could build a submarine, why couldn’t he attempt the same?
Still that week, Zhang gathered 5,000 yuan — about 700 dollars at the time — to buy steel plates, a used motor, and a battery. The work began discreetly. He welded the pieces at night while his wife was away taking care of her mother-in-law.
Without formal training in naval engineering, Zhang based the project on observations of boats and trial and error.
The First Handmade Submarine Cost US$ 700 and Showed Leaks
Six months after starting the work, Zhang completed the first submarine prototype. The vessel measured 6 meters long, about 1.2 meters high, and weighed approximately 2 tons. The vehicle could dive only 1 meter deep and had leaks.
Even with the flaws, Zhang decided to pilot the submarine during initial tests. He described the experience as the realization of a dream, although he admitted he was afraid that the structure would not withstand.
Despite the technical limitations, the prototype attracted enough attention to secure Zhang a national utility model patent in 2016. This type of patent, common in China, recognizes technical solutions deemed new or useful.
In the same year, he registered a second patent related to a surface boat with a propulsion system that reduces wave formation.
Lack of Family Support and Continuation of the Project
Zhang’s family never showed enthusiasm for the submarine project. His wife described the initiative as costly, risky, and of no practical use. Even in the face of criticism, Zhang continued working.
Between 2016 and 2023, he gained practical experience, saved money, and analyzed the flaws of the first prototype. This period was essential for him to begin a second, more ambitious project.
The Second Submarine: Birth of the Big Black Fish
In 2023, Zhang began constructing a new submarine. This project would become the Big Black Fish. The total investment reached 40,000 yuan, about 5,570 dollars, eight times the cost of the first prototype.
The new model had larger dimensions. The hull measured 7 meters in length and 1.8 meters in height, allowing for two passengers side by side. To address the stability issues that affected the first submarine, Zhang adopted a simple solution: he poured about 2 tons of concrete into the bottom of the hull, creating a permanent counterweight.
According to him, the concrete helps maintain the submarine’s balance during dives.
Ballast System Allows the Submarine to Dive and Return to the Surface
The Big Black Fish uses a system of ballast tanks, a technology common in submarines. Zhang installed two tanks — one at the bow and the other at the stern. When these tanks are filled with water, the submarine loses buoyancy and dives. To surface, the water is drained.
Each joint of the hull was manually welded by Zhang himself. He used silicone and industrial adhesives to improve sealing and installed circular hatches that help distribute the water pressure better.
The final result was a submersible made of 5 tons of steel and an approximate displacement of 7 tons, capable of diving up to 8 meters deep. The submarine can navigate at about 4 knots (approximately 7.4 km/h) and stay submerged for up to 30 minutes, using only an electric motor powered by batteries.
The vehicle also has reverse gear, which facilitates maneuvers in narrow areas.
The First Public Dive in the Fengle River
On July 2, 2025, Zhang took the Big Black Fish to the Fengle River, near his home in Hanshan. Residents of the region gathered on the banks to watch the test. Zhang entered through the top hatch, closed the lid, and activated the ballast tanks.
The submarine slowly sank into the murky waters of the river and disappeared for a few moments before resurfacing. To assist in navigation, Zhang installed a waterproof camera attached to a pole about 5 meters long at the bow. The system allows observing the riverbed before diving.
Inside the cockpit, there is a small panel with blue lighting and a non-slip rubber-coated joystick, used to control the direction. During the initial tests, Zhang used the submarine to retrieve a lost fishing net belonging to a local fisherman. For the rescue, he received 3,000 yuan as a reward.
Experts Warn About the Risks of Handmade Submarines
The invention has also raised concerns among experts. Professor Li Zheng, a naval architecture specialist at Shanghai Jiao Tong University, commented on the case in an interview with CCTV.
According to him, maintaining the structural integrity of a submersible vehicle is a complex challenge. Even at a depth of 8 meters, water pressure is already significant. Experts point out that submersibles built without technical certification, appropriate pressurized hulls, or emergency systems for forced ascent can pose serious risks.
So far, no official safety inspection has been released about the Big Black Fish. Zhang claims to be aware of the risks but says he will continue to improve the project.
Rural Inventors and Innovation Outside China’s Largest Technological Centers
Zhang’s story has also drawn attention for contrasting with the more well-known image of technological China. While the country invests billions in hypersonic planes, artificial intelligence, and quantum computing, Zhang’s case illustrates an inventor working almost alone on a complex project.
According to data from the National Intellectual Property Administration of China (CNIPA), the number of utility model patent applications submitted by individual inventors in rural provinces has grown in recent years.
Many of these projects are linked to transport, agriculture, and low-cost solutions. In Anhui, a federal program launched in 2020 offers 5,000 yuan subsidies for rural inventors who keep their creations operational for more than six months.
The Next Project: An Even Larger Submarine
Zhang Shengwu has already stated that he intends to build a third submarine, larger and capable of deeper dives.
The details have not yet been defined, but he claims to continue working the same way he started the original project: without formal technical plans and primarily basing calculations on accumulated experience. According to Zhang, his mind is always thinking about the next project.
Meanwhile, the Big Black Fish remains anchored in the Fengle River. The submarine has no official certification and does not meet traditional naval engineering standards.
Even so, it dives, surfaces, and continues to function — the result of ten years of work by a farmer who decided to build a submarine after watching a television program.
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