Routing a large square in wood?

Hey everyone,

I was after some advice on the best way to halve the depth of a piece of wood, over a large square area. I figure the best way to explain is to actually show what I mean, so below are a CAD interpretation of that.

Attachment 235466 (http://www.woodworkforums.com/attachments/f11/235466-routing-large-square-wood-render2.jpg)

Attachment 235465 (http://www.woodworkforums.com/attachments/f11/235465-routing-large-square-wood-render.jpg)
The reason that I want to do this is because I need the middle of the wood to be thinner,...

Source: http://www.woodworkforums.com/f11/routing-large-square-wood-160012/

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