D.R.I.L.L.

The D.R.I.L.L. ( D iligent  R outer  I n the  L ost  L ands) is a brown semi-blimp that has 1000 HP, moves as fast as a blue bloon, and has the following ability: It spawns 8 Ceramic Bloons when popped, hence the brownish color.
 * Pathfinding: Moves off track and create a new path (it sometimes drill near the surface so the new path can be a tunnel or not, you better hope it's not a tunnel!) from that point to another point from the existing track, and of course the other bloons can move on that new path, too! Oh and it also kill every towers on its way (it deals 20HP damage per second so if you can destroy it quick enough you can save a Temple or some buildings). If the semi-blimp is destroyed before the path is done, the other bloons won't go to the new path, but other D.R.I.L.L.s can continue drilling a half-done path. It only moves as fast as a Red Bloon when drilling. (NOTE: only works on ground terrain! (few!))

Trivia

 * A D.R.I.L.L. once drilled through a giant aerial platform to create the Cloud Graffiti.