I've found three Tony Hawk's Pro Skater 2 levels wraveled into THUG. Plus, a little hidden place:

1) Go to New Jersey. Go to the train station. Stand on top of the train station lip. Look to the left. You will see a flashing icon. Go out and get onto the ledge that its on. This will unlock the Old Skool level. To enter it, go to Change Levels and select Old Skool.

2) Go to Hawai. Go to the shop called the Tiki Shop which has a wooden Tiki outside. Jump into its mouth. You will go down a long tunnel. When you come out, make sure you hit the THPS2 symbol. Then go into the change levels thing and select Venice.

3) Go to Moscow. Hop onto the building which the Christmas tree lights are attached to. Grind along on the side away from the tanks and into the big window. Go up the stairs and get it. Then select Hangar.

4) This is just a secret skatepark. Go to Hawai. Go to the place where the car is. Face the opposite way that it is pointing. Do a Spine Transfer over the ramp. Voila!

Hope you enjoyed this.