TOMB RAIDER ANNIVERSARY'S ON Wii, help?

I am stuck in a level where there's four rooms to get 4 keys, one is called Poseidon where I have to get the key in this room of water and a pillar in the Centre with levers. I get to the top of the room via the pillar, but don't know what to do next??? i just need to get the key behind the gate.... help I had to move on to "underworld" it was SO difficult to complete.....