Tony Hawks Underground [PSX]

Perfect Rail Balance
Enter "letitslide" at the cheats screen.

Perfect Rail Balance - Alternate Code
Enter "SWEETBALANCE" on the cheats codes screen in the options menu.

Cheat Codes
keepitsteady - Perfect Manuals
letitslide - Perfect rail balance
digivid Unlocks - All Thug Movies

These cheats can be turned on or off through the start menu.
Rollerskates - Complete 129/129 Goals.
Kid Mode - Complete 129/129 Goals.
Cool Specials - Complete 129/129 Goals.
Flame (While Grinding) - Complete 129/129 Goals.
Always Special - Complete 129/129 Goals.
Perfect Rail - Complete 129/129 Goals.
Perfect Skitch - Complete 129/129 Goals.
Perfect Manual - Complete 129/129 Goals.
Moon Gravity - Complete 129/129 Goals.

Unlockables
Movie: KISS Concert - Activate K-I-S-S letters on Hotter Than Hell level
Create-a-Deck Mode - Beat "Slamma Jamma" on Story Mode
Hotter Than Hell - Beat story mode on any skill level.
Iron Man - Complete Beginners Story Mode
Gene Simmons - Complete Story Mode on Normal difficulty.
THUD - Complete story mode on SICK difficulty level
Movie: Bails 1 - Find 3 secret tapes
Movie: Bails 2 - Find 6 secret tapes
Movie: Always Hard - Find 9 secret tapes
Pedestrians - Find every GAP in the game
Level: THPS2 Hangar - Find old skool icon on Moscow map
Venice Beach - There is a small smoking Tiki god statue (from the beach) in the back left corner of Hawaii in front of a shop. Ollie into it and hit triangle.
School 2 - There's a red icon in the back 1/4 pipe section of the train station in NJ, get it.