Congratulations on your recent purchase of a property with a pond! Owning such a large pond can be a joy; ducks, birds, fish, frogs, aquatic plants, dragonflies...there is nothing like enjoying a summer day by the pond or just looking out the window and seeing the water shimmering in the sunlight.
Your discription of the swirling pond is great, sounds like a strange science-fiction movie doesn't it! I can just picture a dark hole down below your pond with exotic creatures living in a netherworld of subsurface mud spewing clear swirling liquids into the pond!
What you are seeing, the swirling and moving of the sand being pushed upwards is the effects of natural springs entering your pond. Clear and cool water is coming up through narrow fissures and keeping your pond filled with fresh water. These springs are feeding your pond and they do have a current, a pressure, that pushes water up from below.
Having a natural spring feeding your pond is very good. It will keep your pond water level from dropping and will help support fish species that prefer cooler waters like trout.
Welcome to the wonderful world of ponds and thanks for wrriting.
Samantha
TPR