I've dug a pond, 8ft round, 2ft 6in deep in the middle with a gentle slope. There were water boatmen on it within hours of filling and now in year 2 its got frogs, dragonflies, water skippers and even newts. Beware that the amount of soil that comes out of it is about 3 times the amount of soil that was in it, and here is your chance to fill some raised beds. I don't find it any good as a water reservoir because if the water butts are empty, such a shallow pond is nearly empty too. In fact I filled it from my water butts twice last summer. Well worth the space for the fascination of the wildlife, and the frogs are good slug eaters. I have put a little deck on the side and sit and contemplate when gardening degenerates into wine drinking