Next to the left-hand floor switch are two blocks.

It will topple over and weight down the switch on the floor in front of it. Pull the pillar to the right of the door (facing it) using the grapple. Head round the corner to find a switch in front of a door with two pillars either side of it and two floor switches. Use this to get to a ledge and follow them round the wall onto a bar across more ledges and then back down to the base of the room on the other side of the water. Climb through to the next room with a pool in it and then up a ramp to reach a stone block. Swim down the linear route and then emerge in a dry room. To open the door you need to get the three eye-shaped symbols to line up in the middle. Grab it and place it in the remaining empty hole. In front of the doors on another stone plinth is the other handle. Place the handle you are holding into one of the empty slots at the center of either of the lower circles. Directly in front of you is a large circular metal door Swim inside and through the passage until you find a handle you can pick up on top of a stone plinth, take it and swim back to the tall structure you saw before and through the entrance. Swim round it until you find an entrance. As you face its entrance, turn to the right and notice a clump of stones. If you dive straight ahead and down you'll end up on a stone pathway with a tall stone structure ahead of you.

Tomb raider underworld pic