As rainwater collected carbon dioxide from the atmosphere, slightly acidic rainwater containing small amounts of carbonic acid were capable of eating away at the limestone, forming these towers and islets located in Halong Bay. This is a classic example of Karst's dissolution mechanism. This rain water made its way into the natural cracks and crevasses of the rock, and eventually widened the rock over the course of millennia into caves, tunnels, and bays. This process was amplified in the Vietnamese tropical region. Due to the extra vegetation, the rainwater absorbed extra carbon in its seeping path through the rock, becoming more acidic and better developing this karst environment, making this one of the best visible karst environments in the world.
The visible limestone today has existed for nearly 500 million years. Its first 100 million years of life was spent deep under the ocean, but uplift moved it into a shallow sea environment for an additional 150 million years. Sea level fluctuations allowed for the surfacing of many of these subaqueous limestone formations, allowing for the Karst features seen here. It is for this reason that many of the developed karst regions are underwater, unable to be seen. Now smothered by water from the melting of the last ice age, many unseen valleys and drowned Karst formations remain hidden.
Over the years, more limestone has been eroded than exists today. Halong Bay faces environmental danger today, as mangroves and seagrass beds are being cleared for tourist boats. Fuel and oil have created pollution problems, and portable toilets created for tourists have polluted the surrounding soil and water. Many additional dangers pose a threat to the continued existence of this geological treasure. Hence, efforts are being made to preserve Vietnam's Halong Bay.
Image Credit: Lonely Planet