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.