PC version
You can unlock a Cheat called 'Pea Soup Mode!' then enable it in the secrets menu!This cheat differs in the N64/PC versions. N64 its all green fog but PC it changes colour!
N64 Location:[]
Gardelle County Jail (Helicopter Area) - Go to Dr. Death's Schism Gate and go to where you fight the helicopter and destroy it. Then, go to the tunnel to the right of the entrance. The "Secret Activated" message will appear.
N64 Pea Souper
Dreamcast Location:[]
Temple Of Fire - Pool of blood room with the two blood waterfalls and two different ramps going around the room. Go up the ramp and shoot the second panel to open the passageway. Go inside and then proceed up the big ramp. Jump on the railing to the left and jump towards a hallway in the wall. You should grab the ledge in front of it. Pull yourself up. The "Secret Activated" message will appear.
Alternatve way is to come back when you have the flambeau go to the top of the blood pool room and enter the passageway, use the flambeau on one of the skin doors then it will lead you to the cheat location.
PC and Remastered Location:[]
Go to London Underground via Jack The Ripper. Go to the middle stall in the women's bathroom and the "Secret Activated" message will appear.