How to make a horizontal & vertical website

Built a really simple site that scrolls horizontally for a client.
This can be viewed at http://liosounds.com.au

The client now wants to have sub pages, and the logical plan for me was to keep the horizontal movement and add in vertical movement for subpages. This just keeps a fixed viewable area, and moves the content underneath it. Very sure that I have seen it done, but no idea on how to do it.

Maybe the picture helps to explain it.

Does anyone have a link to a tutorial or something that can point me in the right direction.

Thanks in advance!
Paul