The free expansion to combine the Days of Wonder hits Small World and Small World: Underground should now be available at your friendly local game store. Only one Small World: Tunnels expansion per person and I’ve heard through the grapevine some store might require that you’re a regular customer or you make a purchase to receive the update. I guess it all depends on just how friendly your FLGS may be.
The expansion includes a dozen tiles; half represent entrances to the underworld and the other half are egresses to the surface.
Rule can be found regarding incorporating the update into your games right here!
If there is no store near you, you can also order one directly from the Days of Wonder web store. The Tunnels expansion is free, but unless you also purchase another product you’ll have to pay a small shipping and handling fee.