Modifying 2D array elements

Question

Basically I'm trying to modify an element in a 2D array. How the array is initialized seems to be effecting the behaviour. I'm new to Ruby so I'm not sure how initializing an array would effect this. What am I missing to get the desired result from the desired initialization method?

arr_1 = [[0, 0, 0], [0, 0, 0], [0, 0, 0]] # undesired initialization
arr_2 = Array.new(3, Array.new(3, 0)) # desired initialization

arr_1[0][0] = 99
puts arr_1 # desired result below
=begin
99
0
0
0
0
0
0
0
0
=end

arr_2[0][0] = 99
puts arr_2 # undesired result
=begin
99
0
0
99
0
0
99
0
0
=end