Tunnel

Tunnel is a type of track in BTD games. It has made its first appearance in BTD5(?). All non-MOAB bloons travel through the tunnel by going into it, which protects them from certain attacks and prevents them from being detected by towers.

Starting with BTD6 generation games, MOAB-class bloons no longer travel over the tunnel. Instead, they shrink as they go inside them and return to normal size when coming out of them.