By entering an even combination of A's and N's in different sequences for your password, you can gain access to a variety of levels within the game. Here's a list of password combinations and the levels they take you to:
AANNNNAA: Deep City
NNNAANAA: The Marble Sea
AAAANNNN: Origin Beach
NNNNAAAA: Island Zone
NNNAAAAN: Undercaves
AAAAAAAA: Undercaves
NNNNNNNN: Welcome to The Machine
AAANNNNA: Welcome to The Machine
ANANAANN: The Vents
AANNAANN: The Vents
NNAANNAA: Dark Water
NANANNAA: Dark Water
ANNANNAA: Dark Water
ANANANAN: Hard Water
NANANANA: Hard Water
AANNANAN: Hard Water
NNAANANA: Hard Water
All other combinations will not work, and will transport Ecco back to the starting screen of the game.
The Last Fight: KNLMLMLC
Enter the code SHARKFIN for infinite air and sonar.
Input a valid password, then press Start, and wait for the screen that shows the name of the level you're currently in with your password. Next, hold A and Start. Keep hold of A and Start until Ecco appears on the screen. Now press Start to unpause and you will be invincible.
AJPPOWAX is the password for this level
The Undercaves: UYNAINCC
The Undercaves 2: DQDIACCI
The Vents: FKWLACCA
The Lagoon: NDRBRIKR
Ridge Water: HYAUGFLV
Open Ocean: FNCQWBMT
The Ice Zone: DWFFZBMV
Hard Water Zone: QGDJRQLA
Cold Water Zone: MCLFRQLW