One summary of the legend that emerged bears strong similarities to the King Arthur Lady of the Lake legend, except with turtles:
While fighting against the Chinese, King Le Thai To has in his possession a very valuable sword. After 10 years of continuous struggle, the King finally defeated the Chinese and reclaimed Vietnam's independence. One day, while sailing on lake Luc Thuy, a large turtle appeared. The king drew his sword and pointed at the creature. The turtle immediately grab hold of the sword with its mouth and submerged. The king mourned the lost of such valuable sword, demanded that the lake be emptied and dredged. Both the turtle and the sword were not found. The king realizing that the gods must have lent him the sword to drive back the enemy, but now that Vietnam is free, the sword must be returned. King Le Thai To named the lake Ho Hoan Kiem or Lake of the Returned Sword.
In the Ngoc Son Temple, there is a large preserved turtle that was found in the lake in 1968, proving that giant turtles do live in the lake.