Hồ Hoàn Kiếm (Lake of the Returned Sword) was once a part of the Red river (Song Hong). Through thousands of years of changes in the geography, the lake moved to its present position far away from the river. The lake is the focal point in the old quarter of Ha Noi.

According to the legend, during one of the many conflicts with China in the Viet Nam’s history, heaven sent a magical sword to Emperor Le Thai To, with which he defeated the Chinese army and regained Vietnam's independence. One day, while sailing on the lake (then named Luc Thuy), a large turtle appeared. The king pointed his sword to the turtle. The turtle quickly grab the sword with its mouth and immediately submerged. The king mourned the lost of his magical sword, demanded to empty the lake but even then the turtle and the sword have not been founded.

The Emperor interpreted the incident that heaven must have lent him the sword only to protect the country but now it has to be returned. This is the explanation of the name Ho Hoan Kiem or Lake of the Returned Sword. Later rulers of Viet Nam have all contributed to the beautification of the Hoan Kiem Lake. A pagoda, Ngoc Son, is a very popular place to visit, where also a shrine exists, which is dedicated to general Tran Hung Dao, a national hero responsible for many victories against the Mongols. On the southwest end of the lake lies Tháp Rùa (Turtle Tower).