Carcassonne: Der Tunnel, a small expansion for Carcassonne that appeared in issue 2009/6 of Spielbox magazine, consists of only four tiles and two tokens for each player. In addition to your normal tile placement and optional follower placement, you may place one of your tunnel chips on any exposed tunnel entrance (whether on a tile from Der Tunnel or from another expansion) on the table, regardless of whether that entrance is on the tile you just played. Once both of your tunnel chips have been placed, they connect the two roads, wherever they might be.
Carcassonne: Corn Circles, Plague & Tunnels had been announced as a release by Rio Grande Games for 2012. It would contain three small expansions each previously published on their own: Crop Circles, The Plague, and Tunnels. It has since been cancelled.