In the start room, jump into any of the three pools of water. You'll sink into a flooded room. Push the button on the side wall: the structure in the middle of the room opens up. Swim inside and drop down for a Quad Damage. When you grab it, it will teleport you out.

Get back into the water. You'll notice that there are 4 pools to get up to the air again. The extra one is the entryway to a small secret room.

Get the silver key -the wooden cage opens. Ride the elevator and drop down the hole in the floor. A door will open for a short while, revealing some yellow armour. And a bit of trouble.

After the blue key door, jump across the moving platform over the lava. Watch your step. An elevator will take you down to a rather large room. On the far wall are two buttons. Press the button on the left. The ceiling will start to drop down. There will be a small opening for you, to avoid crushing. Once it's down, jump up onto it and ride it back up. At the top there will be a passage for you to follow to the secret area.