An earthy track composed of 4 pipes connected to a pool of water. These pipes are the bloons' paths. This could be a very hard track since it is composed of 4 paths (entrances and exits). At first, bloons inside the well went out of it, merging into 4 paths, then turn around the pipes the same time as they exit the same time, too. You can build Buccaneers and Subs on that well.
Despite the fact that it has 4 very short paths, it is rated expert and not extreme due to the fact the track isn't short enough to be Extreme.
This is most likely due to the insane difficulty of the track, and Ninja Kiwi wants to give chances to the players, so they don't have to endlessly waste Monkey Money, they can save, and they can test out strategies in Sandbox Mode.
However, due to the differing costs in Monkey Money to play Main Street and Bloontonium Lab or Tar Pits, it would be, in theory, possible to rate it extreme and make it cost nothing or anything lower than 25 to be merciful to a player.