There used to be a ferry moored by the Premonstratensian convent in Zwierzyniec distroct where the trading route crossed the Vistula river. On one winter night, the rope broke, and the ferry went down the river. The sisters, naturally, ordered a new one, yet before it arrived, they were woken up one night by the sound of the ave-bell. When they left their cells in terror, the nuns saw galloping riders: these were merchants pursued by Mongols. The escapers turned their steeds towards the crossing, not knowing that there was nothing to ferry them, and the level of the waters did not allow one a safe crossing. Both the merchants and the Tartars rushed into the waves of the Vistula. Everyone but one merchant drowned.
The man who was saved offered to purchase a bell for the monastery in thanks for being saved. The bell he ordered with a famous founder turned out to be cracked. Two more attempts at casting the bell brought the same result: each was broken. Finally a flawed bell had to be hung in the convent, and tolled every evening, calling sisters to pray for the souls of the drowned with its ten strokes. The tradition continued until 1917, when the Austrians requisitioned the bell to recast it into cannon.
Another version of the legend says that the furious Tartars cut the ropes on which the bell was suspended, and drowned it in the Vistula. Since that day, the waters of the Vistula rise on Midsummer Night, and the bell flows out to the surface. Its sound can be heard until the clock strikes midnight.