If a player can survive until stage 20, 30 and 40 without losing a life, a door icon appears that, if collected in time, transports Bub and/or Bob to one of three secret rooms. Along with 36 diamonds, there is a coded message written on a plaque at the bottom of each room. The first line of this indicates the alphabet ( = A, = B etc. ) and the text that follows is a cryptic message explaining various game tips.
Transliterating the messages, they read as (spelling and grammar as they appear in the game):
- First Secret room (with a Grumple Gromit statue (with his eyes struck out)): "If you want to become the old figure, use the power of your friendship, and fight with me!".
- Explanation: The message essentially tells the players that they have to reach Grumple Grommit to become human. Also, the tone of the message suggests that the messages are written by Grumple Gromit himself.
- Second Secret room (with a big broken Lightning bottle): "I enclose these magical medicine here, because those are my worst fears....".
- Explanation: Grumple Gromit explains his weakness to thunderbolts, which is indeed the weapon that players must use against him; the final level provides the Lightning bottle, which enables the players to breathe lightning bubbles.
- Third Secret room (with a heart under a glass pane): "If you want to get back your love of truth you must help each other until the last....".
- Explanation: Perhaps the most cryptic of the three messages, this refers to the fact that the game cannot be truly "won" unless both players reach the final round and destroy Grumple Gromit together. (It is thus impossible to see the true ending of Bubble Bobble in single-player mode, unless a second player joins in during the Grumple Gromit battle.)
This coded alphabet is used to display the "Super Mode" code (after finishing the game with two players in 'standard' mode), and appears in several other Taito games, like Rainbow Islands and the Puzzle Bobble series.
Secret doors appear on levels 20, 30 and 40, with one that skips you forward 20 levels if you make it to stage 50 without dying. 
Dying inside of a secret room is possible, if killed by the secret's room time up monster, called Rascal. If that happens, the player will lose a life and if that was the player's last life, the game will end by causing the maximum reached round to be "Round 102" for the first secret room, and "Round 103", "Round 104" for the second and third, accordingly.
Performing the previous actions will also trigger a glitch in the first new game started, by causing the player to be transported automatically from round 1 to the first (or second, or third, depending on where the last life was lost in the previous game) secret room. This will also disrupt the order in which secret rounds appear, by making them appear earlier. In the second secret room, the thunderbolts actually seek out the players themselves, and by parking a player to the far right side of the round, will cause the thunderbolts to start striking on the far left side and eventually re-emerging from the right side.
Playing techniques and styles
Bubble Bobble is a game heavily relying on gameplay and precise technique rather than graphics, and it features a series of special techniques and tricks a player can perform to maximize his or her score, make some rounds of the game easier or faster to finish or just to be able to survive or even finish a round.
Some of these techniques have special nicknames, which may differ from player to player and from country to country.
- Kissing monsters or just kissing means killing a monster by blowing a bubble at almost contact distance: the monster will be instantly bubbled and the bubble will be instantly popped, giving the visual effect of the player killing a monster with a "kiss". Some players flip their joysticks in the opposite direction after pressing the bubble buttons, giving more chances of an "instant pop" and changing flight direction for the dead monster. This technique is useful in stages where monsters move too fast, bubbles last for too short a time or it's otherwise hard to bubble them normally. Of course good timing is required for this technique to work.
- Riding bubbles means keeping the jump button pressed when dropping on a bubble: if done correctly, instead of popping the bubble, your dragon will instead jump on it, possibly continuously, enabling him to "ride" bubbles in order to reach otherwise unreachable areas. Some stages can't be finished without this technique.
- Climbing is a step up from riding bubbles. It means standing at half a bubble distance from a wall, jumping and blowing a bubble almost simultaneously, jumping up from that bubble and blowing another bubble and so on. This is necessary if the air current pushes down bubbles but you need to climb up. Having the rapid-bubbling power-up (the yellow candy) makes climbing a lot easier, especially if you got the running shoes already.
- Bubbling oneself through means "riding a bubble" through the opening at the top of a stage or even just through the ceiling of a stage in order to appear at the lower part, like some flying monsters can do. This technique is required to finish some stages or to get unstuck from some places, or just to save time.
- Blowing against the wall means blowing bubbles against wall at contact distance: the bubbles will pop immediately thus giving the player 10 points per bubble pop. This can be used to either increase a player's score, or to set a player's score to a specific amount, in order to do other tricks.
- Two equal digits means using the "blowing against the wall" technique or other score-adjusting techniques in order to make the two penultimate (100s and 10s places) digits of at least one player equal, e.g., 456770, before the last enemy bubble is burst. If done correctly and the score is not modified when this occurs, then all remaining non-special bubbles on screen will be turned to 700-point bonuses, whose appearance depends on the digit picked. E.g., 7 gives Chocolate Ice Creams, 3 gives Hamburgers, and so on.
- This trick is easier to do with two players (one player adjusts his score and the other bursts the bubbles), but it can also be done with only one player, although calculating exactly how much (and if) one's score will be modified when bursting the last enemy bubbles can be extremely complex, if not unpredictable, especially if there are very large and clustered bubble bunches.
- Rounds with numbers ending with 5 and 0 up to and including level 50 generate bonuses from bubbles automatically, though, and some rounds (including round 1) do it by default.