Begin the game and move Ecco left and right. Press Start to pause the game while Ecco is turning. You have to pause it when Ecco is directly facing you. When you do that, press Right, B, C, B, C, Down, C, Up. If you did it right, a cheat menu that has options such as stage select, message test, sound test, and more, will appear.
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
The Island Zone: UWXIOQLK
Deep Water Zone: EILQOQLC
The Marble Sea: XAKUQQLS
The Library: FDGXQQLC
Deep City: ZUVPQQLU
City of Forever: AABBRQLU
Jurassic Beach: PLABUNLT
Pteranodon Pond: FQREUNLI
Origin Beach: QXKIUNLX
Trilobite Circle: OBEMUNLX
Dark Water: JNXPUNLA
Deep Water 2: WSGAKNLC
City of Forever 2: ZBPIGPLD
The Tube: KUVEKMLK
The Machine: SDDBKMLG