A leak with the same cross section as a 3" pipe could empty a 1/2 acre pond in about 1 day. So it doesn't take a real big leak to draw a pond down.
Most leaks aren't round of course. Many times leaks are nothing more than a sandy or gravelly vein in the soil. Some are more like narrow cracks you might see in dry ground. Fast leaks usually don't get better, and often worse. Slow leaks can improve with time, or not.
