Why Vietnam's iconic limestone towers are actually a submerged, 20-million-year-old geological museum

Countries & Cities · Published 29 September 2026 Swipe more like this →

In short: Hạ Long Bay is more than a stunning collection of nearly 2,000 limestone islets. It is a rare, mature karst landscape where ancient towers were partially swallowed by the sea. This unique combination of tropical erosion and marine invasion has created a natural wonder that spans 500 million years of formation.

The landscape of Hạ Long Bay is the result of a 20-million-year evolutionary process. It features a classic 'fenglin' karst topography, where steep, isolated limestone towers rise from the ocean. These towers, which reach heights of 50 to 100 meters, are remnants of a larger landmass where the intervening plains were submerged by the sea. This marine invasion distinguishes the bay from other karst landscapes globally, making it arguably the most extensive example of its kind, second only to Yangshuo in China.

Beneath the surface, the bay acts as a geological record. The limestone itself has been forming for 500 million years, and the area contains evidence of five Quaternary geological cycles. The bay's unique features include 'marine notch' caves—horizontal tunnels carved by seawater dissolution—and enclosed lakes formed within drowned dolines. These caves, such as the massive Hang Dau Go, contain stalactites and stalagmites that have been studied for their role in understanding ancient sea levels and climate history.

Beyond its geology, the bay hosts a complex ecosystem with endemic species like the Livistona halongensis and the recently discovered sponge Cladocroce pansinii. Human history here is equally deep, with archaeological evidence of prehistoric cultures like the Soi Nhụ dating back to 18,000 BC. Today, the area remains a site of both cultural significance and economic transition, as the government works to relocate traditional floating fishing communities to protect the heritage zone while managing the pressures of a growing tourism industry.

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