The Swamp Ruins are found within the Great Swamp of A Link to the Past.
At the south end of the Great Swamp, Link can find the Swamp Ruins, a large Ruin which is the Light World counterpart of the Swamp Palace from the Dark World. Inside, Link will find a Treasure Chest containing some Bombs. In the following room, there are two switches. Pulling the one on the left will drop bombs, while pulling the switch on the right will lower the floodgate, allowing water to flow freely.
Allowing water to flow freely has several effects. Just outside of the ruin, Link will now find a Piece of Heart along with some Fish. Link can pick up this Fish and toss it into the water to earn a Red Rupee. More importantly, opening the floodgate in the Light World will allow Link to progress into the Swamp Palace within the Dark World.
The entrance to the Swamp Ruins in the Light World
The entrance to the Swamp Ruins in the Dark World