Tomb raider underworld?

hey I'm bad at games like this but i enjoy playing. anyway i got this game for the ps2 and guess what I'm already stuck. I'm on the the path to Avalon part were the 3 disc door is but i can't find it. Can you give me a description on how to find it so that I can. Sorry about wasting your time but I'm very grateful to who ever can help me. Thanks Chloe Alexis Snow