Episode 99 - Valley of a Hidden Storm 3 'Training in the Lake!'
It took the children 20 minutes to walk to the 'Lake' for their last training session of the day. On the way, Zene's trainees gave the Paladins some history/information about the supposed 'lake'. And apparently, this 'lake' is connected to the Royal family's lake. This 'lake' would become a 'river' every half a year, and flush out water into the ocean. How they manage to stop the flow of the river and turn it into a lake? … That's classified information.
Anyways, because the water is still, it is called a Lake. And because this lake comes from the Royal Lake, knights would often come to soak and swim to receive 'blessings and luck' from the Royal Lake.