I've played MDK2 HUNDREDS of times.
That is how I came across these: (note: please give me credit for these.)
****CONFIRMED SECRETS****
Level 1:
1. Everyone knows this one: When you reach the first massive room, the one with the alien generator, and fan, there is a room beyond the sniper lock that is set in that circular hole in the wall. Just glide in...
2. After you use all those sniper mortars to bomb the domes, you reach this tunnel, right? exit the tunnel. There is the fan that leads to the LOOOOOOONG vertical pipe that leads to Hanz, the (cue deep echo voice) *FINAL BOSS*.
Ride the fan to the mouth of the vertical pipe, but do not go up! Instead, turn to the tunnel you exited, and glide onto it's roof. You will find... a black hole grenade. Not too useful by now, but it's cool....
3. During the boss fight, you will occasionally see platforms glide by in the distance. These replace any of the lower class aliens you kill during the boss fight. There is a sniper lock underneath the both, and if you are sharp enough, you can shoot the lock to stop the aliens respawning.
Probably more trouble then it's worth though...
Level 2:
4. Another one you'll probably know: Where you begin , the walls to your left and right are destructable.
5. This room is a bit farther in; it has a high platform with 2 bifs on it, and like to alien generators on ledges. On those ledges, there are little nooks in the ceiling with guns and health.
Level 3:
6. Try the fart cheat (L + R + Left + A for the dreamcast veesion) three times in front of the fireplace...
7. In the room with Chuckleberry Finn, light your cigarette lighter underneath one of the standing rockets...
8. After you press the button with Chuckleberry Finn, a wall opens up. Go to the groove of the wall on the right side. There is a door!
Go in and visit a familiar room.
9. This is the part with all that girder jumping. At one point, you will jump on to a brownish pipe, then onto another girder, and contiue along it to the left. Two girders form a stair, and you enter the wall to the left. BUT WAIT! Continue straight past the entrance, and walk along the girder to the end. then, equip the ladder and use it (the ladder anchor is invisible). Walk to the end of the girder you reach, repeat. Repeat once more, and you reach a mr. fizzy jackpot. The same method is used to return.
Level 4:
10. Apparently, there are certain stars in sky that you can zoom in on with your sniper helmet, and you'll see a picture of the developers.
11. You'll reach that room, with two of those really hard to kill aliens. (they are big, have a big gun on their chest, and spines.)
There is also an alien generator that spawns these little puppy things that rush you.
ANYway, You enter this room from a vertical pipe going down, and you are on top of this weird globular structure in the air. there is another one in front of you, with a cloak floating to it's side. This second structure has an entrance on the side facing the wall.
12. There is this place, where you are on the ground, but there is a drop to lower ground beneath you. There is a bif on a ledge to your left, and another one below you. Do not drop down. Kill the bifs, then glide to the corner just to your right. There is a fan there that you can use to reach the left ledge.
(Note: On the lower ground, there is a door to your right. It has an apple, but if you get it, you hear a laugh, and the room spins. There are more doors now, so the exit is unclear, but I think it MAY be to your immediate right. This room is stupid by the way. It has no purpose)
Level 5:
13. Before you drop down to the circular room with the rotating lasers, there are to gat guns on the other side of the girder.
(Note: I haven't played MDK2 in a LONG time, so I'm uncertain about the exact location of these. The re is a small possibility that these guns are in a different, but similar location.)
14. In the labyrinth, there is a metal wall with a crack in it. shoot it for some shotguns.(The wall should be on the way to exit of the labyrinth, you do not have to out of your way to find it.)
15. After the labyrinth, you reach the bridge that breaks. there are many aliens here, and 2 grates on the walls. The right one is easier to reach, but the left one is possible to. You can reach the left one by using the crates on the left, some jut out a little bit.
16. Maybe the worst part of level 5, is the tedious part just before the boss, where you jump on little platforms on the green sludge river and aliens keep spawning. Never fear, thelamebrain is here with a solution: Before you fall, If you saved a little rocket fuel from the previous area, then you can fly over the breaking bridge to the other side. There is some health here, and some guns. Shoot out the wall at the end and jump down. Ta-da!
Everyone knows the telescope, so I won't be going into that.
Level 6:
17. When you are solving the last bomb, you can find some plutonium in the upper left corner.
Level 7:
18: After you run from Shwang's ship, and solve the first room, you reach a room with 3 sniper lock behind wall extensions, and a big pit in the middle. Now look around the wall. Look REAL hard. You'll find a weird little design on the wall near the floor, vaguely resembling a target. Walk into the wall a little bit left of it. You'll go through the wall into a little room full of cool stuff.
19. On the last checkpoint, when you JUST get out of the tunnel, turn left. you'll see a black shape, that sort of resembles a sinking ship.
You can reach it by gliding over. It has lots of cool stuff on it.
20. Also at the last checkpoint, at the exit of the tunnel: There are two poles with a blue dot on top at the sides of the platform. Jump onto either one, and then jump onto the roof of the tunnel. (there's a dummy decoy here!)
You can reach the well known castle in the sky from here. To the right of your original position (still on the roof) fire your chaingun around. you should see it ricocheting off an invisible path. Follow it (It's long) to a yellow square. This is a fan. Glide up (it's long). At the end will be another invisible path you'll have to find. Follow it to the castle. The castle has a wacky picture of the developers behind it.
The giant conehead is well known. I won't be going into that one.
Level 8:
21. At the nuclear tower section, Go right to the well lit section. Somewhere in the floor is an elevator. I don't know it's precise location, but it's there somewhere. (And could someone please tell me how to climb it up? I can't figure it out, and keep having to use the stupid elevator.)
Level 9:
22: There is this place where you must destroy an alien generator to press a button beneath it. There are also some birdbrains hanging out here. You should see a path to the right. Follow it to get a mr.fizzy.
23. There is this puzzle, right? You press the button, and the wall opens up, reveling 2 buttons, press the correct one the wall opens up, revealing 3 buttons,etc. When you finish it, before exiting the area, search the wall to the right of the door. You should find a shape resembling the one in secret 18. It's a door that will open and have cool stuff in it.
24. This one will be a bit vague. There is a maze where you must use your ladder to go up. There is a floor piece close to the right wall I think. It will open if approached revealing some items inside. I don't know the precise way to get to it, sorry.
Level 10:
No secrets known
****UNCONFIRMED SECRETS****
Level 2: This secret was on Bioware's website a long time ago. Apparently, those doors that give you guns and health (but more often, a boxing glove) Have about a 1/200 chance of giving you these really weird weapons, like the "black hole gun..."
Level ?: I've heard a rumor that you can find Waldo somewhere.
The mixed up character cheat: Press START to pause the game. Now hold L+R and press Up, Down, Right, Left, A, Y, B, X, X+START.
Your character will become part Kurt, part Max and part Doctor Hawkins.
