2-dim array mystik

$mod = array(
            0 => array(8, 2, 3, 4, 5, 6, 7),
            1 => array(7, 1, 8, 3, 4, 5, 6),
            2 => array(6, 7, 1, 2, 8, 4, 5),
            3 => array(5, 6, 7, 1, 2, 3, 8),
            4 => array(4, 8, 6, 7, 1, 2, 3),
            5 => array(3, 4, 5, 8, 7, 1, 2),
            6 => array(2, 3, 4, 5, 6, 8, 1),
            7 => array(1, 5, 2, 6, 3, 7, 4)
            );

echo " $mod[3][1] ";

Hvorfor bliver svaret på dette:  Array[1]

Hvordan skal ellers skrive det?