EVO, it is entirely possible your pond was dug near 40' deep originally and due to it's relatively small diameter to depth, the steep sides simply caved in and filled the bottom. Since you say you have no spillway, it could also be "spring" fed and the spring just fluctuates with it's source water table level. Essentially making your pond a groundwater level controlled pond. As the water table rises, water enters your pond and rises...when the water table lowers, your pond drains.
