 Using high-resolution satellite imagery and a mathematical model, scientists have estimated that the mean depth of lakes around the world is substantially lower than previously thought. How many lakes are there, and how deep are they? These are fundamental questions in the study of lake water. With advances in satellite imaging techniques, research over the last decade has narrowed in on the first question, estimating well over 100 million lakes on Earth. The question of depth, however, has remained elusive. A complete understanding of these parameters is critical to understanding the ecological and economic functions of lakes, specifically their role in global carbon cycles. A team of researchers working in Sweden and the US set out to find an accurate means to estimate the volume and depth of Earth's lakes. To accomplish this, they developed and tested a mathematical model which incorporates the Earth's topography and the surface area of 28 million lakes. The team then tested their prediction method against a high confidence database of lakes in the US, Canada and Sweden. The estimates, they found, corresponded quite well with the survey data. Once verified, the researchers then used the mathematical approach to calculate the Earth's overall lake volume and average depth. Their estimates? A volume of nearly 200,000 cubic kilometres and a mean of 42 metres deep. These improved estimates of depth and volume represent a major step forward in understanding the distribution of lake water on Earth. The finding that lakes are significantly less deep on average than previously estimated has important implications for the role of these bodies of water in global geochemical cycles.