Underminer Threat: Finding the gallery item
Go to the third gate that you can break. Then, go backwards until you see a black wall on the left that is cracked. Break the black wall to get the item.

Cheat mode
Enter one of the passwords at the "Secrets" menu:

FROZBOOM - Frozone's Super move
MRIBOOM - Mr. Incredible's Super move
FROZPROF - Frozone gets 1,000 experience points
MRIPROF - Mr. Incredible gets 1,000 experience points
MRIMASTER - Mr. Incredible has everything at max
FROZMASTER - Frozone has everything at max
WHYMUSTWEFIGHT - Unlimited energy
SHOWME - Full gallery unlocked
LEVELLOCKSMITH - Level select
HUDBEGONE - Disable HUD
THISISTOOEASY - Unknown
SHAMELESS - Unknown
ROLLCALL - Unknown
NUKE - Unknown
MAXIMILLION - Unknown
UNSTOPPABLE - Unknown
THISISTOOHARD - Unknown
INAHURRY - Unknown
ICANTHEARYOU - Unknown
HEADROOM - Unknown
EGOPROBLEM - Unknown
DONEB - Unknown
DONEA - Unknown