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I have a nested data structure / JSON, how can I access a specific value?

I am using the US Census API and end up with a two dimensional json array from a jQuery.get() request. My result (data) looks like this:

[[P0010001,NAME,state,county,tract], [2703,Census Tract 4001.01,17,119,400101], [5603,Census Tract 4001.02,17,119,400102], [4327,Census Tract 4002,17,119,400200]]

It looks exactly like a two dimensional javascript array, but I cannot access it like one when I try:

var population = data;

alert(population[1][0]);

Is there a way to convert the json array into a javascript array, or to convert it to a string, which could then be put into an array?