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There are different kinds of tracks that you can unlock in this game. There are sprint tracks, drift tracks, cicuit tracks, and drag tracks. Here are the cheats for each track in the underground (these need to be entered at the main menu):

Circuit: down, R, R, R, X, X, X, Z
Drag: down, R, R, R, X, X, X, Z
Drift: left, left, left, left, right, X, R, Y
Sprint: up, X, X, X, R, down, down, down

You can also unlock the physics for the cars in need for speed underground by hitting R, up, up, up, down, down, down, L

Underground seems like one of the best for the series, so check back for more cheats and codes for this game very soon.