Pottery, porcelain, purple-clay ware, copper coins and wooden planks were among the more than 60 artefacts detected, examined and retrieved from depths of up to 3,000 metres (nearly 2 miles), the report said.

The scale of the survey – conducted from China’s two largest research vessels – was unprecedented, the team said. Using new sonar technology and hardware, they can investigate 100 sq km (38 square miles) each day, detecting objects just a metre (3 feet) long.

Chinese government reveals most significant archaeological finds of 2021

The project’s lead engineering scientist, Chen Chuanxu, an associate researcher with the Chinese Academy of Sciences’ Institute of Deep-sea Science and Engineering, said the team was delving three times deeper than previous surveys, but with greater efficiency thanks to a close collaboration between robots and humans.

Towed sonar is used to locate objects of interest, which are then photographed by an unmanned underwater vehicle.

“If a sunken ship or suspected cultural relics are found in the photos, a manned deep dive will be arranged for underwater verification, confirmation and the extraction of cultural relics,” Chen said.

Video recordings made by the deep-diving crew and broadcast in the CCTV report showed them taking laser scans of a suspected ship’s hull, collecting sediment samples for later analysis and using a mechanical arm to extract fragile items such as pottery and porcelain.

Deng Qijiang, deputy director of the underwater archaeology institute with the National Cultural Heritage Administration in Beijing, told CCTV further investigation was needed to date and identify the ships.

But the latest survey had “established a complete set of deep-sea archaeological investigation workflows, and it will pave the way for more search and discovery of deep-sea underwater cultural relics in the future”, he said.

01:58

Wreckage of Chinese warship the Jingyuan, which sank in 1894, discovered

Wreckage of Chinese warship the Jingyuan, which sank in 1894, discovered

A Beijing-based sonarologist who was not involved in the project, but is familiar with the technology, said the archaeological survey techniques would also be able to discover military assets planted on the seabed by other countries.

The sonarologist, who asked not be named because of the issue’s sensitivity, said the US military had developed a number of deep sea devices that could be deployed in contested regions like the South China Sea.

Among them is a device known as an “upward falling payload” which can be planted on a seabed more than 2,000 metres below the surface and put to sleep for up to five years.

The six-metre device can be activated at any time and release sensors, drones or missiles to the surface where they could collect intelligence, act as decoys, or directly attack enemy targets, according to the US Defence Advanced Research Projects Agency, a major funder of the technology.

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Self-powered soft robot developed by Chinese scientists reaches world’s deepest point

Self-powered soft robot developed by Chinese scientists reaches world’s deepest point

The sonarologist said other military seabed devices could monitor submarine activities, generate signals for underwater positioning, or act as a charging station to power up drones.

“Detecting and removing these devices can cost a tremendous amount of time and money. To the navy it is usually not cost-effective. But the situation can change if the same region interests underwater archaeologists,” he said.

The Chinese government has an ambitious five-year plan to carry out archaeological surveys across the world’s oceans, tracing the ancient Maritime Silk Road from Southeast Asia to Africa by 2025.

Sunken ships of Chinese origin and their cargoes are expected to shed more light on China’s historical status as a maritime power and strengthen its territorial claims to a number of disputed islands.

Zheng conducted seven expeditions to regions as far away as eastern Africa, buying local goods and distributing gifts from the Chinese emperor.