01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly.
02 - Combine cake mix, water, vegetable oil, and eggs. Beat according to package directions. Pour batter into prepared pan and bake for 28-32 minutes until toothpick inserted in center comes out clean.
03 - Remove cake from oven and let cool for 10 minutes. Using handle of wooden spoon, poke holes across entire surface about 1 inch apart, going halfway through cake.
04 - Dissolve lime gelatin in 1 cup boiling water, stirring until completely dissolved. Stir in cold water.
05 - Slowly pour gelatin mixture evenly over cake, ensuring it fills the poked holes.
06 - Refrigerate cake for at least 1 hour to allow gelatin to set completely.
07 - Whip heavy cream with powdered sugar and vanilla extract until stiff peaks form.
08 - Spread whipped topping evenly over chilled cake. Sprinkle with lime zest and garnish with fresh lime slices.
09 - Refrigerate until ready to serve.