The Mainau pond is an artificially created retention basin with dimensions of approx. 20 X 20 m, which is located in the middle of the forest, only approx. 300 m from the entrance to Mainau Island. Here the rainwater is collected from a road above, so that it cannot reach a reed area below. The pond is completely filled with fallen leaves. There are virtually no floating aquatic plants. In one spot, there is an approximately 5 square meter area with a high concentration of iron bacteria, turning the sediment an orange-brown color.