Heat oven to 350° F. Grease a 10-by-10-inch baking pan. In a large mixing bowl, combine the cake mix, eggs, and oil, along with the amount of water called for by the package directions. Beat until well combined. Pour the butter into the bottom of the prepared pan and sprinkle evenly with the sugar. Spread the mangoes evenly over the sugar, then pour the batter over the mangoes. Place in oven and bake until a toothpick inserted into the center of the cake comes out clean, about 45 minutes. Let cool 15 minutes. Unmold onto a serving dish with a lip (to contain the juices).