In a medium bowl combine the sweetened condensed milk and Key lime juice. It will take a bit to combine them well. The juice should be well incorporated.

In a separate bowl beat the whipping cream with an electric mixer until soft peaks form. The tips should curl. Fold whipped cream into lime mixture until thoroughly combined. Spoon filling into graham cracker crust, smoothing evenly. Cover and freeze up to 4 hours or until firm. Remove from freezer about 10 minutes before serving to make serving easier. Return leftovers to freezer, covered.