Small World: Tunnels, a free mini-expansion that debuted at Spiel 2011 in October, allows players to use both Small World and Small World Underground at the same time, using a dozen cavern markers as tunnel entrances to move from one game board to the other. Six tunnel entrances, numbered 1-6, point down from Small World to Underground, while the other six point up to mark the corresponding tunnel exits.
Small World: Tunnels allows for up to six players to compete at the same time, and the rulebook details how to set up the two game boards to accommodate different player numbers, which terrain types are equivalent, and so on. This expansion will subsequently be available at game stores in Europe and the U.S.
The Small World: Tunnels pieces can also be found in the Small World: Realms expansion.