[ruby-talk:11468] multidimenstional arrays?

Hi, I'm a bit new to Ruby, and I have a couple of questions:

1. How do you instantiate multidimensional arrays?
For single dimensions, I can do:
a = Array.new
a[100] = "foo"

But if I wanted to do 2 dimensions, I currently create a one dimensional 
array, and if I wanted to enter a new value, I have to check if the row 
is empty:

a = Array.new
if a[200] = nil then
     a[200] = Array.new
end

a[200][200] = "foo"

Is there a simpler way?
something like
a = [][]
a[300][300] = "goo"
(which doesn't work...)

2. Multiline comments?
Is there a mechanism for it in ruby? ie /* ... */

Thanks for any help