Hidden Achievements

 are a special set of achievements that are not normally visible to the player. These achievements only become visible if the specified requirements are met. These are discovered usually through curiosity or random discoveries. They appear in BSM2 Mobile, BTD6 and BATTD.

Currently known Hidden Achievements in Bloons TD 6
In BTD6, Hidden Achievements appear with the name and description "???" until the achievement is completed. There are currently 12 known Hidden Achievements, which include:

Currently known hidden achievements in Bloons Super Monkey 2 Mobile

 * Allies Assemble - Summon all 6 allies (10000 Gold Blops).
 * Big Spender - Buy an entire tree of weapons in one go (75000 Gold Blops).
 * Breaking Wind - Tap the windmill in Monkey Lane until it breaks (500 Gold Blops).
 * Can't Hide From Me - Pop 10 hidden Bloons on Monkey Lane (2000 Gold Blops).
 * Gettin' Crowded - Have 4 monkeys on screen at once (10000 Gold Blops).
 * Heavy Lifting - Tap the crane until it picks up the scrap (2000 Gold Blops).
 * I'm So Dizzy - Spin Super Monkey in the weapons menu (500 Gold Blops).
 * Look! No hands! - Pop more bloons in a level with a core weapon than L/R (2000 Gold Blops).
 * Monkey Mastery - Buy 100% of weapons for 1 Super Monkey (75000 Gold Blops).
 * Rise and Shine - Wake up Super Monkey after falling asleep (2000 Gold Blops).
 * Sink The Sub - Tap the Sub in Frozen Fields until it submerges (500 Gold Blops).
 * Organized Chaos - Have 5 abilities active at once (3 epics and 2 activated abilities) (10000 Gold Blops).
 * Precision - Get Bronze, Silver, or Gold with exact Bloon pops (100000 Gold Blops).
 * Skull-Diggery - Tap the tar pit until the bones surface (2000 Gold Blops).

Currently known hidden achievements in BATTD

 * By the Light of the Sun God! - Become the Sun God (Done by activiating Supermonkey's Sun God ability, which requires the said upgrade to be bought first, as well as having 4 Super Fans in his range when using the ability) (5 Idols)
 * Awkward... - Have Finn, Flame Princess, and Princess Bubblegum on the map at the same time (also possibly Finn, FP, Warrior PB; Dungeon Finn, FP, PB; or Dungeon Finn, FP, Warrior PB) (1 Uncommon Wish Orb)
 * Old School - Have the Monkey Apprentice, the Sniper Monkey, the Dart Monkey, and the Boomerang Monkey on the map at the same time (1 Uncommon Wish Orb)
 * Treasure Hunter - Buy the Treasure Hunter Upgrade (Buy Captain Cassie's Bloon Pillage upgrade and then have a Supermonkey with the Sun Avatar upgrade in her range) (1 Rare Wish Orb)
 * That's not how Boats work! - Place Cassie on an underwater map (1 Uncommon Wish Orb)
 * Not so BAD - Pop a BAD (Appears on Round 75 in Impoppable Difficulty, or late in Martian Games) (3 Cosmic Essences)
 * No Cheating or you Die - Reach Round 100 (Can only be achieved in Martian Games since normal adventures only have at most 75 Rounds. Note that for the Steam Version of BATTD, this achievement is not in the Steam achievement list, but is still in-game) (1 Epic Wish Orb)

Trivia

 * While 2TC, Bill Greates, and Perfect Paragon are hidden achievements within BTD6, they are not hidden on the Steam achievement list, and are visible like normal achievements.