Begin by crushing graham crackers into fine crumbs. Combine them with melted butter until they resemble wet sand. Press this mixture firmly into the bottom of a 9x13 inch pan.

In a large mixing bowl, whisk together sweetened condensed milk, fresh key lime juice, and lime zest until smooth and creamy, about 3-5 minutes.

Gently fold in thawed whipped topping into the key lime mixture, being careful not to deflate the fluffiness.

Spread half of the key lime filling over the prepared crust. Top with another layer of graham cracker crumbs, followed by the remaining filling. Smooth it out evenly.

Cover the cake with plastic wrap and refrigerate for at least 4 hours or overnight to allow flavors to meld and set.

Before serving, slice the chilled cake into squares and garnish with extra whipped topping and lime slices.