I think there's a switch in the underground hand-shaped area that will open the center grate in the middle room.

In the Northeast section of the main room, there is a crawlspace that you can go through. There is a Crystal Switch in the wall that you have to hit (it's well hidden in the creepy face). Once through, you will be in a room with both a Floor and Wallmaster. You can kill or ignore them and go through the locked door. Then you have to blow up the pile of dirt on the floor in order to reveal a switch that will open the trap door in the floor which you have to fall down in order to hit the switch which will make the chest containing the Lens of Truth appear.