Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the yellow cake mix, 1 cup of water, ½ cup of vegetable oil, and 3 large eggs. Mix until smooth, about 2 minutes. Pour the batter into a greased 9x13-inch baking dish. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. Once baked, allow the cake to cool completely in the pan.

Once the cake has cooled, take a fork and gently poke holes all over the top, spacing them about 1 inch apart.

In a medium bowl, combine the lime-flavored gelatin mix and 1 cup of key lime juice, stirring until dissolved. Carefully pour the lime mixture evenly over the poked cake.

Cover the cake with plastic wrap and refrigerate for at least 3 hours, or overnight for even better flavor.

Once the chilling time is complete, spread 8 ounces of thawed whipped topping evenly over the cake.

For a bright look, sprinkle extra lime zest on top and add lime slices if desired. Cut into squares and serve cold.