Depth of an average body of water - determined by the ratio of the volume of the lake's water mass to its surface area. Accurate average depth figures are available only for well-studied lakes because they must be calculated by dividing the volume of the lake by its surface area. A reliable volume figure requires a bathymetric survey. Therefore, average depth figures are not available for many deep lakes in remote locations. The average lake on Earth has an average depth of 2.85 meters (9.35 feet). The average depth of the world's oceans is 3,790 meters, and the deepest depression is the Mariana Trench (11,516 meters).