The proposal advances the theory that these cities "were the convergence point of the Silk Road at land and sea", said He Yun'ao, a Nanjing University history professor in charge of developing the application.

The concept has won central government support. The proposal will likely be presented next year, He said.

The History of Ming written in the subsequent Qing Dynasty (1644-1911) also hints that Zheng's first expeditions were intended to nab Zhu Di's fugitive nephew. His fleets were unprecedented — more than 300 ships, the largest of which was said to be five times bigger than the one Columbus sailed, carrying nearly 28,000 crew members, including linguists.

They were cities at sea. Zheng sailed with supply ships, horse ships and battleships.

But the most mammoth, at nearly 140 meters long and 60 meters wide, were his seven-sail treasure ships. A replica of a rudder from Zheng's mother ship stretches 10 meters.

Boards were slathered with tung oil, and the ships' bottoms were divided into 13 watertight compartments so that a rupture from an unseen rock or cannonball wouldn't flood the entire hull.

The inspiration for the design — still used around the world today — was said to have come from the inside of a bamboo shoot.

Also displayed are seashells polished into translucent windows, since they proved more waterproof than glass.

None of Zheng's ships sank.

There was also the question of internal security. Zheng's sailors required proper documents to board — in this case, stone plaques with inscriptions no one can decode today.