The ancient history of Kathmandu is described in its traditional myths and legends. According to Swayambhu Purana, present-day Kathmandu was once a huge and deep lake names "Nagdaha" as it was full of snakes. The lake was cut drained by Bodhisatwa Manjusri with his sword and the water was evacuated out from there and he established a city called Manjupattan and made Dharmakar the ruler of the valley land. After sometimes, a demon named Banasur closed the outlet and the valley was again a lake. Then lots Krishna came to Nepal, killed Banasur and again drained out water. He has brought some Gops with him and made Bhuktaman the king of Nepal. 

According to legend, what body of water once existed on the site of Kathmandu?