This is a stream that runs for 5.84 km through downtown Seoul. Originally a natural stream, it was covered with asphalt in 1958 to make a road. Following restoration began in July of 2003 and lasted for two years and three months, it is now being enjoyed as a stream amid downtown. With 22 charming bridges and stepping-stones, and a fabulous night view, there are always tourist and local residents enjoying a walk. You can enjoy the beauty of Cheonggyecheon while having a drink at one of many coffee shops and bars lined along both sides of the stream.
source: Korea Travel Guide