Where the wall meets the waves
At Laolongtou, the "Old Dragon's Head," the Great Wall of China built during the Ming Dynasty (1368–1644) terminates in the Bohai Sea. This section, part of the larger Shanhaiguan defense system, was first constructed in 1381 under the direction of General Xu Da. The most seaward segment, a stone structure designed to extend the fortification directly into the water, was added in 1579 by General Qi Jiguang. What is visible today is largely a reconstruction from the 1980s, as the original fortifications were destroyed in 1900 during the Boxer Rebellion.
The science of Laolongtou lies beneath the waves. The original structure extended approximately 22 to 23 meters into the sea. While the rebuilt wall stands prominently, the original foundations record the history of coastal geomorphology and materials science. At low tide, some of the original granite foundation blocks become visible. These submerged remains are a subject for underwater archaeology, providing direct evidence of the wall's original construction and its subsequent battle with the marine environment. The original builders attempted to create a stable maritime foundation by driving iron piles into the seabed and using massive granite blocks, some weighing 2 to 3 tonnes each.
A laboratory for coastal erosion
The submerged portion of Laolongtou is a long-term case study in coastal erosion and the durability of historical construction materials. The Bohai Sea is a dynamic environment, with constant wave action, tidal currents, and sediment transport physically abrading the structure. The global mean sea level has risen approximately 21 to 24 centimeters since 1880, a change that has permanently altered the shoreline and submerged the wall's original base.
The construction materials themselves are central to this scientific story. The Ming Dynasty builders famously used a mortar that incorporated glutinous rice flour with lime, a composite that created a strong, water-resistant binder for the stone and brick. Analysis of these surviving materials in a saline environment provides data on their long-term resilience. The granite blocks used for the foundation were sourced locally, a common practice in the Great Wall's construction. The interaction between the acidic nature of the rice-based mortar and the alkaline seawater, along with the physical stress from waves, shows the decay processes affecting coastal heritage sites. Archaeological investigation of these drowned sections shows the engineering methods used to combat the sea, including the use of cast-iron dovetail joints to bind granite slabs.