The first index refers to the outer array, the next to the next inner
array, etc.

If you don't know in advance how long each array is, then you must check
 lengths before accessing an element.

For example, to access arr[1][2][3], do this:

if (arr.length >= 2) {
    String [][] arr0 = arr[1];
    if (arr0.length >= 3) {
        String [] arr1 = arr0[2];
        if (arr1.length >= 4) {
            String arr2 = arr1[3];
            // use the value here
        }
    }
}

This is ugly. I'm sure you can find a more efficient way to do it.