In an era devoid of navigation systems and weather forecasts, how did sailors venture into the treacherous South China Sea?

Six hundred years ago, fishermen from Tanmen Town in Hainan, China, braved the waves, becoming the only group in world history to continuously explore Xisha and Nansha Islands.

They relied on two treasures: the compass and the unique manual passed down through generations among the Tanmen people, Geng Lu Bu (Book of Seaways). With the compass, the ship could maintain a steady course; with the book, fishermen could sail without losing their way at sea.

The word Geng refers to distance, serving as an ancient unit for calculating maritime journeys. In the bountiful South China Sea, each unit of distance conceals both allure and peril. With names like Maozhu, Huangshanma, and Duanjie, the book designated over 100 islands in the South China Sea in the local dialect of Tanmen, along with more than 200 navigational routes. This textual maritime chart enabled fishermen to grasp the locations and routes of various islands, offering fishing advice such as “never sail in June when the clouds are red” and “check the weather before heading out, observe the tides when setting sail.” In an era without GPS, Tanmen fishermen relied on this generationally refined and continually updated navigation guide to navigate the open seas.

For over 600 years, Geng Lu Bu has not only guided South China Sea fishermen into deeper waters but also to neighboring countries and regions surrounding the South China Sea. With the continuous advancements in navigation and positioning technologies, Geng Lu Bu has become a national-level intangible cultural heritage. No longer just a maritime secret for fishermen, it now stands as a symbol of spirit and collective memory, passed down through generations by the fishermen of Tanmen.