Underground is designed after a cave mine underground. Bloons moved on 2 interconnected rail in a lightbulb shape cross each other at the same time, making the map has 2 entraces and 2 exits. The map is pretty devious, as the paths are quite short. There are 22 blue crytals (8 to the left without counting the one in the minecart and 14 to the right), 8 red bats (3 on the left, 5 on the right and all of them hung upward but not on the ceiling), a minecart filled with crytals (6 crytals to be accurate), about 15 or so little pebbles scattered around the cave, a dusty cave floor with many parts of the ground are cracked and big rocks that haven't been mined yet surrounded the barrier of the map, and the railroad.
- Main article: Underground/Strategies
- Before Version TBA, the soundtrack used Tribes and Tribulations, but since then it uses Sunshine Serenade.