To make the crust by hand: Place in a plastic bag and use a rolling pin to crush.
In a medium bowl, combine crumbs and sugar; stir in melted butter.
To make the crust in a food processor: Process crackers and sugar until crackers are finely crushed, about 20 seconds; add melted butter and pulse until incorporated, about 10 pulses.
Press mixture into a 9 inch pie pan using the back of a spoon.or your fingers.
Bake crust until just set, about 6 minutes (crust will only color slightly).
Meanwhile, in a medium bowl, lightly whisk together egg yolks with the sweetened condensed milk; gradually beat in the lime juice (mixture will thicken) and the zest; set aside.
In the bowl of a stand mixer (or a large bowl and an electric mixer) beat egg whites until foamy; add the cream of tartar and beat until stiff peaks form; set aside.
In a saucepan, stir together sugar and water, then heat over medium heat until the sugar dissolves; reduce heat to a simmer and cook, stirring occasionally, until the soft ball stage (236 degrees F); remove from heat.
With the mixer on low speed, gently pour syrup over the egg whites until incorporated; increase heat to high and beat 2 minutes.
Remove 2 cups of the meringue, then cover the remainder with plastic wrap and set aside.
Fold the 2 cups of meringue into the lime mixture; pour int crust.
Bake (at 350 degrees F) until set, 15 minutes.
Spread remaining meringue over entire surface of pie and sprinkle with powdered sugar (to create extra crunch), if desired.
Return pie to oven for 5 minutes, then turn on broiler and cook 20 seconds to 1 minute until meringue is golden.
Cool pie at least 4 hours before serving; use a wet, thin knife to cut.
Store, uncovered, in the refrigerator, up to 3 days.