The Underground Expansion Pack contains 4 dice that you can use in two ways. With the Standalone mode, you use them instead of the normal Route dice, to build underground networks of Subway Routes and Water Pipes to score points in a whole new way. With the Expansion mode, you use them together with the normal Route dice, but without mixing them! You need two boards for each player, one for the Surface and one for the Underground. The basic Route dice are used on the Surface board (following the normal rules), while the Underground dice (and rules) are used on the Underground board!
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