In one underground crypt in Tergaron, there is a grate that leans up above an open pit. You need to close the grate to pass. I see a small dot on the far wall on the far side of the pit, but nothing I throw triggers the grate to close. Also throwing heavy rocks at the grate causes them to get stuck, but even after throwing 300 heavy rocks at the grate does not cause it to close. How can i close the grate?