On Underground, Bloons enter from the bottom-left and the top-right and exit at the bottom-right and top-left, respectively. The cave walls block line-of-sight slightly. With RBS values of 12.9 and 13.2 for the top and bottom lanes, respectively, Underground's lanes are among the shortest among those of Advanced maps, only bested by Erosion, Mesa's bottom lane, and Cargo's MOAB lane.
Strategy
Despite its short length and the awkward targeting that towers may experience, the abundance of spots that cover both lanes pull Underground down into near-average in terms of difficulty out of the Advanced maps. The center of the map and the spaces to its top & bottom are great spots for most towers, as they can widespread coverage of both lanes when placed there. Stalling support towers, such as Relentless Glue, are especially useful on this map, due to its short length. Towers that take good advantage of Underground's layout are listed below.
When placed at the left-most or right-most spot in the central space, Sauda defeats the first few rounds and simplifies the early-game. In the late-game, she has solid coverage of both lanes, and does large amounts of damage when paired with stunning support towers such as Relentless Glue.
The path layout of Underground allows Glaive Lord's rangs to repeatedly bounce between the lanes, dealing massive amounts of damage. The spinning rangs very effectively can clean up both lanes very effectively if Glaive Lord is placed in the central space.
All of Carpet of Spikes's ability spikes can see action on every round, greatly enhancing its power. It breaks apart the outer layer of ZOMGs, takes care of DDTs, and trivializes the BAD. Pair it with a tower that performs well for clean-up, such as Glaive Lord and The Bloon Solver.
Solver can clean up Super Ceramics with ease, and its puddles create a chain effect that easily clears out lower-tier blimps as well. It pairs especially well with Carpet of Spikes, which pops ZOMGs, DDTs, and the BAD, Solver's main weaknesses, and allows Solver to trigger its puddles and their chain effect.
The intersecting paths are great for the Prince of Darkness, making it an amazing midgame that can saveup for towers with higher cost; these can carry the lategame then.
Easter eggs
Interacting with the bats causes them to fly away, then return after a few minutes.
Easter theme
Halloween theme
Holiday theme
Island statue
