Level 5 - Rising Sun (7 secrets)
1. JUMP off boat. GO to "door/gate" or roughly where the boat started from in the first place (face West) and SINK to the bottom. You should see a hole in the base of the wall leading to a tunnel. (hole in wall). GO through tunnel onto other side. Get cookie.
2. In room where there is a firepit, STEP ON table. Picture in the next room will open. Get flashbomb, grenade launcher.
3. Behind waterfall, SHOOT gong, panel on side opens. Get missile launcher.
4. In corridor with cobwebs hanging down, SHOOT one of the 2 dragon lamps/torches and you should see a "wooden" panel. USE it and get night vision goggles.
5. STAND on top of waterfall, SQUAT and GO towards wall. Get...drumroll please....suspense....A RAIL GUN!!! Prepare to get ERASED!! Now you can test out your waterbottle..I mean rail rods.
6. In room with cobwebs and a painting. There also should be an irritating statue shooting fireballs at you when you leave this room. JUMP into painting. Get missiles.
7. In room with statue in the middle of some water. STAND ON statue's base. Door opens. Get nuke. (Note: If there is another door blocking your way, then Sorry I forgot to mention that BOTH "Buddha" statues (See below notes for location of second one) need to be SUNKEN down, so go step on the base of both statues and make sure they are down when you leave them.
Non-secret things to note:
In the corner there is a pagoda with a "buddha statue" in the middle, STAND ON the statue's base it should sink a little. then get off and STAND ON its base again. It should rise back. Then stand on the statue, yes I mean ON the statue's head! (Balance on one foot like the karate kid...Oops! sorry, got carried away.) Anyway you should be able to see a kevlar armour as well as something that looks like an overgrown waterbottle backpack! (sorry, its rail gun rods! no offence to the artists, that's what I though it was when I first saw it. WHOOOO WANTA SUM WAAATER??? HA! HA! Eh, ehm, enough of that!)
Corridor near cheat #4, has a crack in it, BLOW it open to get a cookie.
Stand on stairs next to waterfall and JUMP on opposite ledge to get uzi, cookie, medikit.
Level 6 - Killing Fields (5 secrets)
1. Fall from waterfall, take a left (go around), get armour.
2. Corridor behind house with fan over the door. BLOW open crack in wall to get heart.
3. On the left side of the "buddha statue", SHOOT the gong above. Get missiles, grenade shells.
4. Hallway with darts, centre of room to the right, there is a nuke. To get to this all you have to do is JUMP with your face rubbing against the wall and you should be able to get it. Easy when you know how.
5. Next to second waterfall (not the one mentioned in secret #1), its by itself in a corner of the map. STAND on green (supposedly moss) platforms/steps and JUMP into open chamber. Get Kevlar armour.
(Source: http://www.pcgameworld.com/ Website)