High Island Reservoir

Imagine standing on a long, grey dam that stretches across a narrow channel of sea. On one side, the calm, blue water of the reservoir stretches far into the distance. On the other side, the open ocean crashes against giant rocks made of stone columns that look like a giant's honeycomb. This is the East Dam of the High Island Reservoir, one of the most amazing places in Hong Kong.

The High Island Reservoir is the biggest reservoir in Hong Kong — it can hold about 281 million cubic metres of water. That is enough to fill more than 112,000 Olympic-sized swimming pools! The reservoir was built to make sure Hong Kong had enough fresh water to drink, especially during dry times. Before it was built, Hong Kong sometimes had to turn the water off for only a few hours a day. The construction started in 1969 and took ten years to finish. Workers built two huge dams to block off the sea, turning a channel of salt water into a giant freshwater lake.

What makes the rocks so special?

If you visit the East Dam, you will see something truly incredible. The cliffs and islands around the reservoir are made of thousands of six-sided rock columns. They look like a bundle of giant pencils or a honeycomb carved from stone. These are called hexagonal rock columns, and they formed about 140 million years ago. Back then, a supervolcano erupted in this area. Thick, hot ash and lava cooled down very slowly. As it cooled, it cracked into these perfect six-sided shapes. The columns here are some of the largest and best-preserved in the whole world. Some are as tall as a 20-storey building!

How did they build a dam in the sea?

Building the High Island Reservoir was a huge engineering challenge. Workers had to build the dams right in the ocean. First, they dug a deep trench on the seabed. Then they filled it with tons of rock and sand to make a strong foundation. On top of that, they placed giant concrete blocks called dolosse. Each dolos looks like a spiky toy jack, and they lock together to stop the waves from washing the dam away. The East Dam is famous for its huge dolosse — each one weighs as much as a small car! A special road was built just to bring all the materials and machines to this remote part of Sai Kung.